cmd/go/internal/modload: prevent tidy downgrading disambiguating modules

If an indirectly required module does not provide any packages needed
to build packages in the main module but is needed to disambiguate
imports, 'go mod tidy' may keep an indirect requirement on that module
to prevent it from being downgraded. This can prevent the introduction
of new ambiguities. This also ensures tidy keeps sums needed to load
all packages.

Fixes #47738

Change-Id: Ib8e33422b95394707894cda7cfbb71a4b111e0ed
Reviewed-on: https://go-review.googlesource.com/c/go/+/344572


Trust: Jay Conrod <jayconrod@google.com>
Run-TryBot: Jay Conrod <jayconrod@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Bryan C. Mills <bcmills@google.com>