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.

    Reviewed-on: https://go-review.googlesource.com/c/gofrontend/+/192718

From-SVN: r275358