Make spack clean env-aware

[scheibelp]

spack clean <spec> will now resolve specs based on the active environment if one is active.

If an env is active but no matching spec is found, this will fall back on fully concretizing.

[scheibelp]

It was pointed out to me that spack clean can be confusing if you mix a list of specs with --all, e.g.

spack clean zlib --all

The zlib part is redundant with --all, because --all will remove all build stages. The user in question assumed that this meant "clean everything up for zlib", but ended up clearing out stages they didn't want to by accident. This can be addressed by either:

• (preferred for me) Forbid specifying --all or any of the other -options together with a list of specs
• Documenting this better

[scheibelp]

I confirmed the added tests fail without the core changes, but the test is somewhat fragile since there are other concretization pathways, like concretize._concretize_specs_together_new.

I could add a dont_concretize_at_all conftest fixture, which I think would be more robust and generally useful.