Use list rather than set when getting specs from YAML file (spec.yaml)

Context

Currently there seems to be cases where two specs, with different dag-hashes, share the same internal hash - returned by hash(spec). This produces some problems when installing from binaries and using the hash to specify the package to be installed, example:

spack  buildcache install /123456789

Let's say there are two YAML files with the same name, version and compiler but different hashes such as:

linux-centos7-x86_64-gcc-6.2.0-packageA-1.0-987654321-spec.yaml
linux-centos7-x86_64-gcc-6.2.0-packageA-1.0-123456789-spec.yaml

I don't fully understand how this is even possible, but there are cases where the specs produced out of these two YAML files can have the same internal hash.

As a consequence, using a set in the implementation of get_specs will consider only one of the two specs with the same internal hash but different dag-hash, thus leading to a problem when the user tries to install the excluded spec.

This could be also solved by implementing a __hash__ method in Spec (which would call to dag_hash() in order to use the same hash employed to concretize the spec), however this commit warns not to use such implementation due to inefficient side-effects.

I have tried to implement this __hash__ method and it certainly slows down the performance of the spack spec command, although not as much as before:

With __hash__ implemented on Spec

time spack spec dealii

real	0m9.371s
user	0m6.975s
sys	0m0.316s

Without:

real	0m7.244s
user	0m6.911s
sys	0m0.316s

The impact might be bigger on other packages so I would just use a list here rather than adding a hash method to the Spec class.

@tgamblin @gartung