cmd/go/internal/toolchain: avoid importing modcmd

cmd/go/internal/toolchain: avoid importing modcmd

modcmd is a high-level command, but toolchain is a low-level building
block. A dependency from toolchain on modcmd makes it very difficult
to call from other lower-level packages without creating an import
cycle.

Instead, use modfetch.Download in place of modcmd.DownloadModule.

For #57001.

Change-Id: I9694706d7225b269f26dc68814894613a3329abb
Reviewed-on: https://go-review.googlesource.com/c/go/+/499316
Run-TryBot: Bryan Mills <bcmills@google.com>
Auto-Submit: Bryan Mills <bcmills@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Russ Cox <rsc@golang.org>