go/build: rewrite TestDependencies to be cleaner, more correct

TestDependencies defines the dependency policy
(what can depend on what) for the standard library.

The standard library has outgrown the idea of writing
the policy as a plain map literal. Also, the checker was
ignoring vendored packages, which makes it miss real
problems.

This commit adds a little language for describing
partial orders and rewrites the policy in that language.

It also changes the checker to look inside vendored
packages and adds those to the policy as well.

This turned up one important problem: net is depending
on fmt, unicode via golang.org/x/net/dns/dnsmessage,
filed as #40070.

This is a test-only change, so it should be appropriate
even for the release freeze, especially since it identified
a real bug.

Change-Id: I9b79f30761f167b8587204c959baa973583e39f2
Reviewed-on: https://go-review.googlesource.com/c/go/+/241078
Run-TryBot: Russ Cox <rsc@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>