This unwinds a small part of CL 31668: we now accept "./." in cleanImport.

Fixes #18778.

Change-Id: Ia7f1fde1cafcea3cc9e0b597a95a0e0bb410a3ed
Reviewed-on: https://go-review.googlesource.com/35646
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Russ Cox <rsc@golang.org>