refactor/rename: fix spurious conflict report when renaming x/foo to y/foo

There is no need to check for import conflicts when the package name
doesn't change.  Add test.

Also, when reporting a (non-spurious) import conflict, make clear that
the error is just a warning.  Add a test for same.

Change-Id: Idde0483b502cd041fd893230fec06b8533f54f3c
Reviewed-on: https://go-review.googlesource.com/24917
Reviewed-by: Michael Matloob <matloob@golang.org>
2 files changed