runtime: make the lock rank DAG make more sense

This groups, comments, and generally reorganizes the lock rank graph
description by subsystem. It also introduces several pseudo-nodes that
more cleanly describe the inherent layering of lock ranks by
subsystem.

I believe this doesn't actually change the graph, but haven't verified
this.

For #53789.

Change-Id: I72f332f5a23b8217c7dc1b21411631ad48cee4b0
Reviewed-on: https://go-review.googlesource.com/c/go/+/418718


TryBot-Result: Gopher Robot <gobot@golang.org>
Run-TryBot: Austin Clements <austin@google.com>
Reviewed-by: Michael Pratt <mpratt@google.com>