`spack gc` has so far been a global or environment-specific thing.
This adds the ability to restrict garbage collection to specific specs,
e.g. if you *just* want to get rid of all your unused python installations,
you could write:
```console
spack gc python
```
- [x] add `constraint` arg to `spack gc`
- [x] add a simple test
Signed-off-by: Todd Gamblin <tgamblin@llnl.gov>
d763d6f738