cmd/go: report 'go get' errors for absolute paths outside module root

'go get' will now check absolute paths without wildcards the same way
it checks relative paths. modload.DirImportPath may be used for both
without converting path separators.

Fixes #38038

Change-Id: I453299898ece58f3b5002a5e80021d6bfe815fdd
Reviewed-on: https://go-review.googlesource.com/c/go/+/226857


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