[dev.cmdgo] cmd/go: sort roots when joining multiple main module roots

When concatenating multiple main modules' roots we need to resort
the list of main module roots. This avoids a panic from unsorted main
module roots. This will get better when we have structured module
roots.

Change-Id: I68fed47b0f3b131ed3dadf45db3c442286a0ced0
Reviewed-on: https://go-review.googlesource.com/c/go/+/338111


Trust: Michael Matloob <matloob@golang.org>
Run-TryBot: Michael Matloob <matloob@golang.org>
Reviewed-by: Jay Conrod <jayconrod@google.com>