Currently, `spack solve` has different spec selection semantics than `spack spec`.
`spack solve` currently does not allow specifying a single spec when an environment is active.
This PR modifies `spack solve` to inherit the interface from `spack spec`, and to use
the same spec selection logic. This will allow for better use of `spack solve --show opt`
for debugging.
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Co-authored-by: Todd Gamblin <tgamblin@llnl.gov>
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