refactor/rename: fix crash with "." as -from package

Cause: the absolute cwd, not ".", must be passed to build.Import.

Also: simplify the lookup logic in findFromObjects, and handle nil
gracefully.

(No unit test---not worth faking os.Getwd.)

Fixes issue #13866

Change-Id: I7e8eede55bc83d98e2ef5e42609c287e74c04501
Reviewed-on: https://go-review.googlesource.com/18453
Reviewed-by: Michael Matloob <matloob@golang.org>
2 files changed