cmd/go: refactor modload.ListModules to accept bit flags

Instead of accepting bool flags, ListModules now accepts ListMode, a
set of bit flags.

Four flags are defined. listRetracted is split into ListRetracted and
ListRetractedVersion to avoid ambiguity with -u, -retracted, and
-versions.

For #40357

Change-Id: Ibbbe44dc1e285ed17f27a6581f3392679f2124fb
Reviewed-on: https://go-review.googlesource.com/c/go/+/306331


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