cmd/distpack: rename go.mod to _go.mod in toolchain modules

cmd/distpack: rename go.mod to _go.mod in toolchain modules

Modules cannot contain go.mod files except at the root
(and we don't keep one at the root). Rename the other go.mod
files to _go.mod.

dl2mod, which we used to convert all the old releases,
did this renaming, but it was missed when porting that
code to distpack.

For #57001.
Fixes #60847.

Change-Id: I4d646b96b5be15df3b79193e254ddc9b11cc8734
Reviewed-on: https://go-review.googlesource.com/c/go/+/503979
Auto-Submit: Russ Cox <rsc@golang.org>
Run-TryBot: Russ Cox <rsc@golang.org>
Reviewed-by: Heschi Kreinick <heschi@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Bryan Mills <bcmills@google.com>