imports: ignore case and hyphens when lexically filtering candidate dirs

This means a directory /gopath/src/foo/Go-Bar is considered as maybe
containing package "bar" or even "gobar".

Also, more tests.

Fixes golang/go#16402

Change-Id: I14208d738e3a081cb6d9bcd83d777280e118f4e7
Reviewed-on: https://go-review.googlesource.com/25030
Reviewed-by: Andrew Gerrand <adg@golang.org>
2 files changed