cmd/go: examine dependencies of main modules in workspace mode

To make sure that we properly pull in everything in all, because
different main modules can interfere with each others' pruning.

Fixes #49763
Change-Id: I0756993d8ae9919ccb27ec460d579d348c38ec3b
Reviewed-on: https://go-review.googlesource.com/c/go/+/370663


Trust: Michael Matloob <matloob@golang.org>
Run-TryBot: Michael Matloob <matloob@golang.org>
Reviewed-by: Bryan Mills <bcmills@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>