avoid non-cachable packages in cache

[nerdvegas]

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.