cmd/go: match go-import package prefixes by slash

cmd/go: match go-import package prefixes by slash

The existing implementation for path collision resolution would
incorrectly determine that:

    example.org/aa

collides with:

    example.org/a

This change splits by slash rather than comparing on a byte-by-byte
basis.

Fixes: #15947
Change-Id: I18b3aaafbc787c81253203cf1328bb3c4420a0c4
Reviewed-on: https://go-review.googlesource.com/23732
Reviewed-by: Ian Lance Taylor <iant@golang.org>
Run-TryBot: Ian Lance Taylor <iant@golang.org>