Suppose we import anotherPackage:M, which exports things from
anotherPackage:Internal. Then GHC will want to read
anotherPackage:Internal.hi.

However, if we have also SOURCE-imported thisPackage:Internal then
we don't want GHC to try to read anotherPackage:Internal.hi-boot
instead.

The mapping that tells us whether a module is SOURCE-imported uses just
the module name for the key, so we have to check the package ID before
looking it up.

Fixes #3007.