cmd/go: remove auto-deriving module path for github.com

This fix removes the special case of auto-deriving the module path
only for VCS github.com. Error message now explicitly requests
the module path. Documentation and its FAQ do not need an update
as only the beginning of the message is mentioned and is not modified.

Fixes #27951

Change-Id: Icaf87a38b5c58451edba9beaa12ae9a68e288ca1
Reviewed-on: https://go-review.googlesource.com/c/go/+/172019


Reviewed-by: Daniel Lublin <daniel@lublin.se>
Reviewed-by: Bryan C. Mills <bcmills@google.com>
Run-TryBot: Jay Conrod <jayconrod@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>