cmd/go: work out VCS information once per repository

We need VCS status information for each main package we load.
If two main packages are under the same VCS repository,
we can reuse that information to avoid duplicating work.

For instance, the kubernetes holds 51 main packages in its root module,
meaning that "go list ./..." repeated the same git calls 51 times.
Instead, use a global par.Cache to deduplicate that work.

Below are the numbers on kubernetes 5eb584d1cb6917,
via "benchcmd -n 8 KubernetesListPackages go list ./...":

	name                    old time/op         new time/op         delta
	KubernetesListPackages          8.91s ± 0%          3.33s ± 1%  -62.61%  (p=0.000 n=7+8)

	name                    old user-time/op    new user-time/op    delta
	KubernetesListPackages          11.2s ± 1%           8.1s ± 2%  -27.50%  (p=0.000 n=7+8)

	name                    old sys-time/op     new sys-time/op     delta
	KubernetesListPackages          8.02s ± 0%          1.67s ± 6%  -79.21%  (p=0.001 n=6+8)

	name                    old peak-RSS-bytes  new peak-RSS-bytes  delta
	KubernetesListPackages          127MB ± 2%          123MB ± 7%     ~     (p=0.328 n=8+8)

Fixes #49582.

Change-Id: Ib7ef5dc7a35c83a11e209441f5d6f3b8da068259
Reviewed-on: https://go-review.googlesource.com/c/go/+/365394


Trust: Daniel Martí <mvdan@mvdan.cc>
Trust: Dominik Honnef <dominik@honnef.co>
Run-TryBot: Bryan C. Mills <bcmills@google.com>
Reviewed-by: Bryan C. Mills <bcmills@google.com>
TryBot-Result: Go Bot <gobot@golang.org>