cmd/go: make 'mod verify' use multiple CPUs

'go mod verify' checksums one module zip at a time, which is
CPU-intensive on most modern machines with fast disks. As a result, one
can see a CPU bottleneck when running the command on, for example, a
module where 'go list -m all' lists ~440 modules:

	$ /usr/bin/time go mod verify
	all modules verified
	11.47user 0.77system 0:09.41elapsed 130%CPU (0avgtext+0avgdata 24284maxresident)k
	0inputs+0outputs (0major+4156minor)pagefaults 0swaps

Instead, verify up to GOMAXPROCS zips at once, which should line up
pretty well with the amount of processors we can use on a machine. The
results below are obtained via 'benchcmd -n 5 GoModVerify go mod verify'
on the same large module.

	name         old time/op         new time/op         delta
	GoModVerify          9.35s ± 1%          3.03s ± 2%  -67.60%  (p=0.008 n=5+5)

	name         old user-time/op    new user-time/op    delta
	GoModVerify          11.2s ± 1%          16.3s ± 3%  +45.38%  (p=0.008 n=5+5)

	name         old sys-time/op     new sys-time/op     delta
	GoModVerify          841ms ± 9%          865ms ± 8%     ~     (p=0.548 n=5+5)

	name         old peak-RSS-bytes  new peak-RSS-bytes  delta
	GoModVerify         27.8MB ±13%         50.7MB ±27%  +82.01%  (p=0.008 n=5+5)

The peak memory usage nearly doubles, and there is some extra overhead,
but it seems clearly worth the tradeoff given that we see a ~3x speedup
on my laptop with 4 physical cores. The vast majority of developer
machines nowadays should have 2-4 cores at least.

No test or benchmark is included; one can benchmark 'go mod verify'
directly, as I did above. The existing tests also cover correctness,
including any data races via -race.

Fixes #38623.

Change-Id: I45d8154687a6f3a6a9fb0e2b13da4190f321246c
Reviewed-on: https://go-review.googlesource.com/c/go/+/229817


Run-TryBot: Daniel Martí <mvdan@mvdan.cc>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Bryan C. Mills <bcmills@google.com>