cmd/go: add cache verification mode

cmd/go: add cache verification mode

If GODEBUG=gocacheverify=1, then instead of using the cache to
avoid computations, the go command will do the computations and
double-check that they match any existing cache entries.
This is handled entirely in the cache implementation; there's no
complexity added to any of the cache usage sites.

(As of this CL there aren't any cache usage sites, but soon there will be.)

Also change GOCMDDEBUGHASH to the more usual GODEBUG=gocachehash=1.

Change-Id: I574f181e06b5299b1d9c6d402e40c57a0e064e74
Reviewed-on: https://go-review.googlesource.com/75294
Run-TryBot: Russ Cox <rsc@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: David Crawshaw <crawshaw@golang.org>