A consequence of CL 19747 was that ad hoc packages of a single filename
are no longer valid -scope arguments, which broke most of the tests.
This CL fixes the tests by extracting the import path from the filename.
The test output now includes the import path, not the package
name, of each of these packages, so the output was updated too.
Also, the -scope argument caused the main package to be imported twice,
once as main.go and once under its import path, causing duplicate
'implements' results.
I'm not sure how I failed to notice this.
Change-Id: I6fb3ae3e1497f0eb8cf88f453660b9e2fd9718d8
Reviewed-on: https://go-review.googlesource.com/19886
Reviewed-by: Michael Matloob <matloob@golang.org>
