cmd/go/internal/modfetch: make Repo.Zip write to an io.Writer instead of a temporary file

This will be used to eliminate a redundant copy in CL 145178.

(It also decouples two design points that were previously coupled: the
destination of the zip output and the program logic to write that
output.)

Updates #26794

Change-Id: I6cfd5a33c162c0016a1b83a278003684560a3772
Reviewed-on: https://go-review.googlesource.com/c/151341


Run-TryBot: Bryan C. Mills <bcmills@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Russ Cox <rsc@golang.org>