internal/dag: add a Graph type and make node order deterministic
The go/types package doesn't care about node ordering because it's
just querying paths in the graph, but we're about to use this for the
runtime lock graph, and there we want determinism.
For #53789.
Change-Id: Ic41329bf2eb9a3a202f97c21c761ea588ca551c8
Reviewed-on: https://go-review.googlesource.com/c/go/+/418593
Reviewed-by: Michael Pratt <mpratt@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Run-TryBot: Austin Clements <austin@google.com>