cmd/go/internal/modfile: don't use cmd/internal/diff
cmd/go/internal/modfile: don't use cmd/internal/diff

This is a partial revert of CL 203218.

cmd/go/internal/modfile is about to be deleted and replaced with
golang.org/x/mod/modfile in CL 202698. cmd/internal/diff is not
visible from golang.org/x/mod/modfile, and it doesn't make sense to
extract it into a new package there.

Updates #31761

Change-Id: I3bbbc4cae81120020e1092c1138524729530b415
Reviewed-on: https://go-review.googlesource.com/c/go/+/204103
Run-TryBot: Jay Conrod <f3fadaedc813344da5ec13b41e26596889cc5baa@google.com>
Reviewed-by: Bryan C. Mills <1c8aad60184261ccede67f5c63a0d2a3bf3c9ff4@google.com>

golang/go,akutz/go,CAFxX/go,cloudflare/go,nawawi/go,golang/go,golang/go,CAFxX/go,akutz/go,golang/go,cloudflare/go,akutz/go,golang/go,CAFxX/go,akutz/go,cloudflare/go,cloudflare/go,cloudflare/go,akutz/go,nawawi/go,cloudflare/go,nawawi/go,CAFxX/go,cloudflare/go,nawawi/go,cloudflare/go,akutz/go,golang/go,CAFxX/go,akutz/go,nawawi/go,golang/go,golang/go,CAFxX/go,CAFxX/go,akutz/go,nawawi/go,nawawi/go,CAFxX/go,nawawi/go