cmd/go: allow configuring module cache directory with GOMODCACHE

Adds a GOMODCACHE environment variable that's used by cmd/go to determine the
location of the module cache. The default value of GOMODCACHE will be
GOPATH[0]/pkg/mod, the default location of the module cache before this change.

Replace the cmd/go/internal/modfetch.PkgMod variable which previously held the
location of the module cache with the new cmd/go/internal/cfg.GOMODCACHE
variable, for consistency with many of the other environment variables that
affect the behavior of cmd/go.  (Most of the changes in this CL are due to
moving/renaming the variable.)

The value of cfg.GOMODCACHE is now set using a variable initializer. It was
previously set in cmd/go/internal/modload.Init.

The location of GOPATH/pkg/sumdb is unchanged by this CL. While it was
previously determined using the value of PkgMod, it now is determined
independently dirctly from the value of GOPATH[0].

Fixes #34527

Change-Id: Id4d31d217b3507d6057c8ef7c52af1a0606603e4
Reviewed-on: https://go-review.googlesource.com/c/go/+/219538


Run-TryBot: Michael Matloob <matloob@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Bryan C. Mills <bcmills@google.com>
Reviewed-by: Jay Conrod <jayconrod@google.com>