Refactor code as a follow-up to
https://go-review.googlesource.com/c/tools/+/154742.
Also, change every instance of "source.URI()" to "fromProtocolURI", so
that we can add a better implementation of that later on (for Windows
support).
Change-Id: Ifa24ffd7e1aebf1f7d05df6f65742769ead0922f
Reviewed-on: https://go-review.googlesource.com/c/154741
Reviewed-by: Ian Cottrell <iancottrell@google.com>
Run-TryBot: Rebecca Stambler <rstambler@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>