Both packagestest and the gopls regtests need to write module data to
the filesystem in proxy structure. Since this seems like a common and
self-contained concerned, factor this out into a shared package.
Change-Id: I5275dbc0cd7b13290061e8bb559d6dd287fbb275
Reviewed-on: https://go-review.googlesource.com/c/tools/+/227841
Run-TryBot: Robert Findley <rfindley@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Rebecca Stambler <rstambler@golang.org>
