os/user: rename group cgo file

In another CL, I'll add a pure Go implementation of lookupGroup and
lookupGroupId in lookup_unix.go, but attempting that in one CL makes
the diff too difficult to read.

Updates #18102.

Change-Id: If8e26cee5efd30385763430f34304c70165aef32
Reviewed-on: https://go-review.googlesource.com/37497Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>