Allow wrapping of PackageContext

Turn PackageContext into an interface so that build systems can wrap it
to add more custom helpers.

This does introduce an API change, though it should be fairly simple.
NewPackageContext used to provide an opaque *PackageContext struct, now it
provides a PackageContext interface.

Change-Id: I383c64a303d857ef5e0dec86ad77f791ba4c9639