cmd/go: don't refuse to update a present package only because of its source configuration

If a package is checked out in the right place, but uses, for instance,
an unusual Git remote configuration, don't refuse to update just because
cmd/go can't parse it. In most cases, `git pull -ff-only` (or the
equivalent in other VCSes) works just fine.

Updates #25432.

Change-Id: I1952a0e6e03f185f34029b186e1756c9549689e1
Reviewed-on: https://go-review.googlesource.com/113456


TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-by: Andrew Gerrand <adg@golang.org>
Reviewed-by: Rob Pike <r@golang.org>