avoid non-cachable packages in cache

The following scenario is possible:
* pkg A is cached
* the rezconf is updated to set pkg A to NON-cachable
* a new resolve picks up cached A

We could add a step that checks if the pkg is still cachable, after it finds a cached copy. If it isn't, the cached copy is not used.

Related - if `rez-pkg-cache --clean` is run, and it finds a cache entry for a pkg that is no longer cachable, it should delete that cache entry.

Trace the resolver path that uses a cached package, then inspect the `rez-pkg-cache --clean` handling of cache entries. Verify both paths check whether the package remains cachable, and confirm that non-cachable packages are neither reused nor retained by cleanup.

Written by the indexing model from the issue text.