External package not always being used resulting in same package being concretized differently

### Steps to reproduce

I'm seeing some strange differences in concretization where a package is being concretized differently. The only difference being an external `pkg-config`. Sometimes the external `pkg-config` is used and for other packages `pkgconf` is being built from scratch resulting in the same packages being built twice with different hashes.

I was able to reduce this behavior with a simple example.

I have a Spack environment with an external `pkg-config`:

```
packages:
  pkg-config:
    externals:
    - spec: pkg-config@0.27.1
      prefix: /usr
```

A bundle with a single package:

```
class NcioBundle(BundlePackage):
    version('1.0.0')
    depends_on('ncio')
```

And a simple spec list:

```
  specs:
  - ncio-bundle
  - ncio
``` 

You can see the full concretization below but the only difference is `pkgconf` and `pkg-config`. 

I have my environment activated and I run `spack concretize` and the same spec concretizes differently:

```
^pkgconf@1.8.0%intel@2021.3.0 arch=linux-centos7-cascadelake
^pkg-config@0.27.1%intel@2021.3.0+internal_glib 
```

You can see when part of a bundle it's using the external `pkg-config`, but if not it's building `pkgconf`.

In my real use-case I have several bundles being built and two contain this `ncio` package and they being concretized differently exactly like my example.

I can solve this issue by setting `pkgconfig` `buildable: False`, or if I modify the built-in `packages.yaml` to prefer `pkg-config`, or override it in my own `packages.yaml`

https://github.com/spack/spack/blob/57f5c92a39a6d3e71be86de7ee98e1e65e2173a8/etc/spack/defaults/packages.yaml#L49


But why would `curl` concretize differently because one is in a bundle? Why isn't my external `pkg-config` always being used?

### Error message

Full concretization below, notice how the only difference is `pkg-config` and `pkgconf`.

```
==> Concretized ncio-bundle
 -   7isq7nk  ncio-bundle@1.0.0%intel@2021.3.0 arch=linux-centos7-cascadelake
[+]  giuwrww      ^ncio@1.0.0%intel@2021.3.0~ipo build_type=RelWithDebInfo arch=linux-centos7-cascadelake
[+]  zq5gb6h          ^cmake@3.22.1%intel@2021.3.0~doc+ncurses+openssl+ownlibs~qt build_type=Release arch=linux-centos7-cascadelake
[+]  qzmzrqg              ^ncurses@6.3.20211021%intel@2021.3.0~symlinks+termlib abi=6 arch=linux-centos7-cascadelake
[+]  wqpzmd4              ^openssl@1.1.1n%intel@2021.3.0~docs certs=system arch=linux-centos7-cascadelake
[+]  di4fmna          ^intel-oneapi-mpi@2021.3.0%intel@2021.3.0~external-libfabric~ilp64 arch=linux-centos7-cascadelake
[+]  qjwlh4i          ^netcdf-fortran@4.5.3%intel@2021.3.0~doc+pic+shared arch=linux-centos7-cascadelake
[+]  jzc2wgy              ^netcdf-c@4.7.4%intel@2021.3.0+dap~fsync~hdf4~jna+mpi+parallel-netcdf+pic+shared patches=2c88dfbd6d339a0336a43b14a65a1d1df995b853b645e4af612617612a642a53 arch=linux-centos7-cascadelake
[+]  3u4dxij                  ^autoconf@2.69%intel@2021.3.0 patches=7793209b33013dc0f81208718c68440c5aae80e7a1c4b8d336e382525af791a7 arch=linux-centos7-cascadelake
[+]  y4uegic                      ^m4@1.4.16%intel@2021.3.0+sigsegv arch=linux-centos7-cascadelake
[+]  vnjncy4                      ^perl@5.16.3%intel@2021.3.0~cpanm+shared+threads patches=0eac10ed90aeb0459ad8851f88081d439a4e41978e586ec743069e8b059370ac,3bbd7d6f9933d80b9571533867b444c6f8f5a1ba0575bfba1fba4db9d885a71a arch=linux-centos7-cascadelake
[+]  cn4xdnq                  ^automake@1.16.5%intel@2021.3.0 arch=linux-centos7-cascadelake
[+]  m62w6ev                  ^curl@7.80.0%intel@2021.3.0~gssapi~ldap~libidn2~librtmp~libssh~libssh2~nghttp2 tls=openssl arch=linux-centos7-cascadelake
[+]  h3n2aln                      ^pkg-config@0.27.1%intel@2021.3.0+internal_glib patches=49ffcd644e190dc5efcb2fab491177811ea746c1a526f75d77118c2706574358 arch=linux-centos7-cascadelake
[+]  ioczwxv                      ^zlib@1.2.11%intel@2021.3.0+optimize+pic+shared arch=linux-centos7-cascadelake
[+]  j5nqr2n                  ^hdf5@1.12.1%intel@2021.3.0~cxx+fortran+hl~ipo~java+mpi+shared~szip+threadsafe+tools api=default build_type=RelWithDebInfo arch=linux-centos7-cascadelake
[+]  b7fa37y                      ^numactl@2.0.14%intel@2021.3.0 patches=4e1d78cbbb85de625bad28705e748856033eaafab92a66dffd383a3d7e00cc94,62fc8a8bf7665a60e8f4c93ebbd535647cebf74198f7afafec4c085a8825c006,ff37630df599cfabf0740518b91ec8daaf18e8f288b19adaae5364dc1f6b2296 arch=linux-centos7-cascadelake
[+]  ycssfzg                          ^libtool@2.4.6%intel@2021.3.0 arch=linux-centos7-cascadelake
[+]  at325hc                  ^parallel-netcdf@1.12.2%intel@2021.3.0~burstbuffer+cxx+fortran+pic+shared arch=linux-centos7-cascadelake

==> Concretized ncio
 -   xfpfe3x  ncio@1.0.0%intel@2021.3.0~ipo build_type=RelWithDebInfo arch=linux-centos7-cascadelake
[+]  zq5gb6h      ^cmake@3.22.1%intel@2021.3.0~doc+ncurses+openssl+ownlibs~qt build_type=Release arch=linux-centos7-cascadelake
[+]  qzmzrqg          ^ncurses@6.3.20211021%intel@2021.3.0~symlinks+termlib abi=6 arch=linux-centos7-cascadelake
[+]  wqpzmd4          ^openssl@1.1.1n%intel@2021.3.0~docs certs=system arch=linux-centos7-cascadelake
[+]  di4fmna      ^intel-oneapi-mpi@2021.3.0%intel@2021.3.0~external-libfabric~ilp64 arch=linux-centos7-cascadelake
[+]  nitel6p      ^netcdf-fortran@4.5.3%intel@2021.3.0~doc+pic+shared arch=linux-centos7-cascadelake
[+]  jueep3f          ^netcdf-c@4.7.4%intel@2021.3.0+dap~fsync~hdf4~jna+mpi+parallel-netcdf+pic+shared patches=2c88dfbd6d339a0336a43b14a65a1d1df995b853b645e4af612617612a642a53 arch=linux-centos7-cascadelake
[+]  3u4dxij              ^autoconf@2.69%intel@2021.3.0 patches=7793209b33013dc0f81208718c68440c5aae80e7a1c4b8d336e382525af791a7 arch=linux-centos7-cascadelake
[+]  y4uegic                  ^m4@1.4.16%intel@2021.3.0+sigsegv arch=linux-centos7-cascadelake
[+]  vnjncy4                  ^perl@5.16.3%intel@2021.3.0~cpanm+shared+threads patches=0eac10ed90aeb0459ad8851f88081d439a4e41978e586ec743069e8b059370ac,3bbd7d6f9933d80b9571533867b444c6f8f5a1ba0575bfba1fba4db9d885a71a arch=linux-centos7-cascadelake
[+]  cn4xdnq              ^automake@1.16.5%intel@2021.3.0 arch=linux-centos7-cascadelake
[+]  m62w6ev              ^curl@7.80.0%intel@2021.3.0~gssapi~ldap~libidn2~librtmp~libssh~libssh2~nghttp2 tls=openssl arch=linux-centos7-cascadelake
[+]  5j7y6ul                  ^pkgconf@1.8.0%intel@2021.3.0 arch=linux-centos7-cascadelake
[+]  ioczwxv                  ^zlib@1.2.11%intel@2021.3.0+optimize+pic+shared arch=linux-centos7-cascadelake
[+]  uxvdtkv              ^hdf5@1.12.1%intel@2021.3.0~cxx+fortran+hl~ipo~java+mpi+shared~szip+threadsafe+tools api=default build_type=RelWithDebInfo arch=linux-centos7-cascadelake
[+]  b7fa37y                  ^numactl@2.0.14%intel@2021.3.0 patches=4e1d78cbbb85de625bad28705e748856033eaafab92a66dffd383a3d7e00cc94,62fc8a8bf7665a60e8f4c93ebbd535647cebf74198f7afafec4c085a8825c006,ff37630df599cfabf0740518b91ec8daaf18e8f288b19adaae5364dc1f6b2296 arch=linux-centos7-cascadelake
[+]  ycssfzg                      ^libtool@2.4.6%intel@2021.3.0 arch=linux-centos7-cascadelake
[+]  at325hc              ^parallel-netcdf@1.12.2%intel@2021.3.0~burstbuffer+cxx+fortran+pic+shared arch=linux-centos7-cascadelake
```

### Information on your system

* **Spack:** 0.16.1-6587-6f2df33
* **Python:** 3.9.7
* **Platform:** linux-centos7-cascadelake
* **Concretizer:** clingo

### General information

- [X] I have run `spack debug report` and reported the version of Spack/Python/Platform
- [X] I have searched the issues of this repo and believe this is not a duplicate
- [X] I have run the failing commands in debug mode and reported the output

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External package not always being used resulting in same package being concretized differently #29930

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External package not always being used resulting in same package being concretized differently #29930

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