internal/task: update golang.org/x dependencies in untagged repos

It's useful to automatically update to latest golang.org/x dependency
versions in all golang.org/x repos, no matter whether the repo itself
happened to have started its own tagging process or not yet. So start
doing the update (and not tag) for the remaining golang.org/x repos.

Add a new 'Wait' field for module dependencies. It tracks whether that
dependency should be waited for during the planning stage. It will end
up true for all repos that are being tagged today, short of those that
are intentionally excluded by the tagx:ignore comment.

Also ignore repos we don't intend to tag when doing the cycle check.
Updating them is a leaf task with no further dependencies, so they're
never going to contribute to a deadlock.

Fixes golang/go#56530.

Change-Id: I3092b1a02d077b70958304bbec8d0e37cedf3ee3
Reviewed-on: https://go-review.googlesource.com/c/build/+/487615
LUCI-TryBot-Result: Go LUCI <[email protected]>
Reviewed-by: Dmitri Shuralyov <[email protected]>
Reviewed-by: Heschi Kreinick <[email protected]>
Auto-Submit: Dmitri Shuralyov <[email protected]>