cmd/go/internal/modload: in readonly mode, do not read go.mod files missing checksums

This was causing test failures while revising CL 293689: splitting the
roots out from the rest of the module graph may allow 'go list -e' to
proceed even when the rest of the module graph can't be loaded (e.g.
due to missing go.mod checksums). If that occurs, it becomes more
important that the moduleInfo helper function itself avoid fetching
unchecked files.

For #36460

Change-Id: I088509eeb3008cc6e8bfe85a00ec2bf500bf9423
Reviewed-on: https://go-review.googlesource.com/c/go/+/301371


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