Delete temp files we aren't using.

We don't want to be leaving lots of unused temp files in the cache
directories ...
More work on not leaving files we don't want

We don't want the cache filled with hundreds of files that we aren't
ever going to use ...
Rewrite LookupImportPath for performance

Turns out LookupImportPath was quite slow ... so let's try a different
way ...
Add caching for m.pkg
Try and get more stuff cached
Caching support for genInterfaces
Use the cache for pkg.Rewrite