moved ResolvePackage to context.

All package lookups are now scoped by package, and the files and
dependant imports are scoped by the specifications of the context.
Don't need to pass explicit context.

All packages now have a reference to their context, and thereby
their Project. This simplifies the information passed to the
various build Targets.
small cleanup
Move context methods to package.

Now that Package has a reference to its Context, we no long need to
pass it to the helper methods on Context.