compiler: only import variable into . if same package
compiler: only import variable into . if same package

If we dot-import a package, we should only add an imported variable to
the package bindings if the variable is in the package being imported.
A test case for this is the 1.13 os package, in which ErrClosed and
friends are defined both locally and in the imported internal/oserror package.

Change-Id: I43e1a08cafa5e6136a82194d32f3deb23bec7d69
Reviewed-on: https://go-review.googlesource.com/c/gofrontend/+/192718
Reviewed-by: Cherry Zhang <d62e63aa42ce272d7b6a5055d97e942b33a34679@google.com>

golang/gofrontend,golang/gofrontend,golang/gofrontend,golang/gofrontend