cmd/go: allow list of absolute directory path

Relative directory paths have always worked.
This CL makes absolute directory paths be handled the same way.
(It was an oversight that they were excluded.)

It also fixes the case of naming the directory holding source code
for a package in a module dependency.

Fixes #14177.
Fixes #26550.

Change-Id: I29a0ca2795d35eca773121ee91a97628b56947ce
Reviewed-on: https://go-review.googlesource.com/126715


Run-TryBot: Russ Cox <rsc@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Michael Matloob <matloob@golang.org>
Reviewed-by: Bryan C. Mills <bcmills@google.com>