cmd/go: add a 'go mod tidy' mode that only verifies whether changes are needed

[bcmills]

Maintainers of large projects will often want to ensure that their module definitions are both complete and minimal at commit time (example: #27643 (comment)).

go mod tidy is the obvious command to verify that a definition is complete and minimal, but it makes changes even when -mod=readonly is set (#26850). As far as I am aware, there is no way to verify (nondestructively) that a definition is minimal — only that it is complete (go list all -mod=readonly), and even then only subject to the current GOOS, GOARCH, and build tags.

If we decide not to change the behavior of go mod tidy -mod=readonly, I think it's important that we provide some other command that would be suitable for use in a commit hook (along the lines of gofmt -l).

[kaedys]

I rather like the idea of simply adding the -l flag to go mod tidy and have it work the exact same as it does for gofmt (ie. doesn't modify the source files, instead printing out where it would have).

[rsc]

go mod tidy -n seems fine. (The go command already uses -n for these things.)

[rsc]

Dup of #27005?

[bcmills]

Dup of #27005?

Yep, looks like it.