The ASP based solver prefers building a package over using externals if it has non-default variant values

This error has been reported by @bvanessen If an external package is declared with `buildable: true` and with non-default choices for variant values, Spack prefers to build a new package instead of using the external. What a user would expect instead is that the external is used whenever it is a viable choice.

### Steps to reproduce the issue
Using this `packages.yaml`:
```yaml
packages:
  openmpi:
    buildable: True
    externals:
    - spec: openmpi@4.1.0~cuda+cxx+cxx_exceptions~java~memchecker+pmi~sqlite3~static~thread_multiple~wrapper-rpath fabrics=psm schedulers=slurm
      prefix: /usr/tce/packages/openmpi/openmpi-4.1.0-gcc-8.3.1
```
and trying to concretize anything that depends on `openmpi` Spack tries to build a new version of the package:
```console
$ spack -C . solve hdf5 ^openmpi
==> Best of 0 answers.
==> Optimization: [0, 0, 0, 0, 30, -2, 1, 0, 0, -29, 0]
hdf5@1.10.7%gcc@10.1.0~cxx~debug~fortran~hl~java+mpi+pic+shared~szip~threadsafe api=none arch=linux-ubuntu18.04-broadwell
    ^openmpi@4.0.5%gcc@10.1.0~atomics~cuda~cxx~cxx_exceptions+gpfs~internal-hwloc~java~legacylaunchers~lustre~memchecker~pmi~singularity~sqlite3+static~thread_multiple+vt+wrapper-rpath fabrics=none patches=60ce20bc14d98c572ef7883b9fcd254c3f232c2f3a13377480f96466169ac4c8 schedulers=none arch=linux-ubuntu18.04-broadwell
        ^hwloc@2.4.1%gcc@10.1.0~cairo~cuda~gl~libudev+libxml2~netloc~nvml+pci+shared arch=linux-ubuntu18.04-broadwell
            ^libpciaccess@0.16%gcc@10.1.0 arch=linux-ubuntu18.04-broadwell
                ^libtool@2.4.6%gcc@10.1.0 arch=linux-ubuntu18.04-broadwell
                    ^m4@1.4.18%gcc@10.1.0+sigsegv patches=3877ab548f88597ab2327a2230ee048d2d07ace1062efe81fc92e91b7f39cd00,fc9b61654a3ba1a8d6cd78ce087e7c96366c290bc8d2c299f09828d793b853c8 arch=linux-ubuntu18.04-broadwell
                        ^libsigsegv@2.12%gcc@10.1.0 arch=linux-ubuntu18.04-broadwell
                ^pkgconf@1.7.3%gcc@10.1.0 arch=linux-ubuntu18.04-broadwell
                ^util-macros@1.19.1%gcc@10.1.0 arch=linux-ubuntu18.04-broadwell
            ^libxml2@2.9.10%gcc@10.1.0~python arch=linux-ubuntu18.04-broadwell
                ^libiconv@1.16%gcc@10.1.0 arch=linux-ubuntu18.04-broadwell
                ^xz@5.2.5%gcc@10.1.0~pic arch=linux-ubuntu18.04-broadwell
                ^zlib@1.2.11%gcc@10.1.0+optimize+pic+shared arch=linux-ubuntu18.04-broadwell
            ^ncurses@6.2%gcc@10.1.0~symlinks+termlib arch=linux-ubuntu18.04-broadwell
        ^libevent@2.1.12%gcc@10.1.0+openssl arch=linux-ubuntu18.04-broadwell
            ^openssl@1.1.1k%gcc@10.1.0~docs+systemcerts arch=linux-ubuntu18.04-broadwell
                ^perl@5.32.1%gcc@10.1.0+cpanm+shared+threads arch=linux-ubuntu18.04-broadwell
                    ^berkeley-db@18.1.40%gcc@10.1.0~docs patches=b231fcc4d5cff05e5c3a4814f6a5af0e9a966428dc2176540d2c05aff41de522 arch=linux-ubuntu18.04-broadwell
                    ^gdbm@1.19%gcc@10.1.0 arch=linux-ubuntu18.04-broadwell
                        ^readline@8.0%gcc@10.1.0 arch=linux-ubuntu18.04-broadwell
        ^numactl@2.0.14%gcc@10.1.0 patches=4e1d78cbbb85de625bad28705e748856033eaafab92a66dffd383a3d7e00cc94,62fc8a8bf7665a60e8f4c93ebbd535647cebf74198f7afafec4c085a8825c006 arch=linux-ubuntu18.04-broadwell
            ^autoconf@2.69%gcc@10.1.0 arch=linux-ubuntu18.04-broadwell
                ^help2man@1.47.16%gcc@10.1.0 arch=linux-ubuntu18.04-broadwell
                    ^gettext@0.21%gcc@10.1.0+bzip2+curses+git~libunistring+libxml2+tar+xz arch=linux-ubuntu18.04-broadwell
                        ^bzip2@1.0.8%gcc@10.1.0+shared arch=linux-ubuntu18.04-broadwell
                            ^diffutils@3.7%gcc@10.1.0 arch=linux-ubuntu18.04-broadwell
                        ^tar@1.32%gcc@10.1.0 arch=linux-ubuntu18.04-broadwell
            ^automake@1.16.3%gcc@10.1.0 arch=linux-ubuntu18.04-broadwell
        ^openssh@8.5p1%gcc@10.1.0 arch=linux-ubuntu18.04-broadwell
            ^libedit@3.1-20210216%gcc@10.1.0 arch=linux-ubuntu18.04-broadwell
```

### Error Message

There's no error, but the optimal solution a user would expect is:
```console
$ spack -C . solve hdf5 ^openmpi
==> Best of 0 answers.
==> Optimization: [0, 0, 0, 7, 0, -2, 1, 0, 0, -2, 0]
hdf5@1.10.7%gcc@10.1.0~cxx~debug~fortran~hl~java+mpi+pic+shared~szip~threadsafe api=none arch=linux-ubuntu18.04-broadwell
    ^openmpi@4.1.0%gcc@10.1.0~atomics~cuda+cxx+cxx_exceptions+gpfs~internal-hwloc~java~legacylaunchers~lustre~memchecker+pmi~singularity~sqlite3~static~thread_multiple+vt~wrapper-rpath fabrics=psm patches=60ce20bc14d98c572ef7883b9fcd254c3f232c2f3a13377480f96466169ac4c8 schedulers=slurm arch=linux-ubuntu18.04-broadwell
    ^zlib@1.2.11%gcc@10.1.0+optimize+pic+shared arch=linux-ubuntu18.04-broadwell
```

### Information on your system

* **Spack:** 0.16.1-1920-ddce89e4ce
* **Python:** 3.6.9
* **Platform:** linux-ubuntu18.04-broadwell
* **Concretizer:** clingo

### Additional 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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The ASP based solver prefers building a package over using externals if it has non-default variant values #22596

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The ASP based solver prefers building a package over using externals if it has non-default variant values #22596

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