cmd/go/internal/modget: support the suffix '@patch' in 'go get'

As of this change, an explicit '@patch' suffix is to '-u=patch' as
'@latest' is to '-u'.

RELNOTE='go get' in module mode now supports the version suffix '@patch'.

Fixes #26812

Change-Id: Ib5eee40de640440f7470d37a574b311ef8a67f67
Reviewed-on: https://go-review.googlesource.com/c/go/+/167747


Run-TryBot: Bryan C. Mills <bcmills@google.com>
Reviewed-by: Jay Conrod <jayconrod@google.com>