cmd/internal/moddeps: use -mod=readonly instead of -mod=mod

TestAllDependencies is attempting to check that the modules in GOROOT
satisfy certain properties; it should not modify those modules itself.

The “quick” part of the test checks that vendored packages are present
and complete, without constructing a parallel GOROOT. It shouldn't
resolve new dependencies or change formatting in any way.

The longer version of the test already constructs a parallel GOROOT
and tidies the modules within it. That part of the test will flag any
modifications needed to the go.mod and go.sum files, without modifying
the original GOROOT.

From what I can tell, the failure mode in #46695 is caused by running
the test on a module rooted in $GOROOT proper. There is no such module
in the mainline Go repo, but it may have been introduced in the fork
and could also be introduced by stray edits in contributor CLs. It
should be diagnosed clearly.

For #46695

Change-Id: I62b90ccbd54cb3e3b413017021c952a7b1d455e7
Reviewed-on: https://go-review.googlesource.com/c/go/+/328770
Trust: Bryan C. Mills <[email protected]>
Run-TryBot: Bryan C. Mills <[email protected]>
TryBot-Result: Go Bot <[email protected]>
Reviewed-by: Dmitri Shuralyov <[email protected]>

Author: Bryan C. Mills

src/cmd/internal/moddeps/moddeps_test.go

@@ -68,7 +68,7 @@ func TestAllDependencies(t *testing.T) {
 
 			// There is no vendor directory, so the module must have no dependencies.
 			// Check that the list of active modules contains only the main module.
-			cmd := exec.Command(goBin, "list", "-mod=mod", "-m", "all")
+			cmd := exec.Command(goBin, "list", "-mod=readonly", "-m", "all")
 			cmd.Env = append(os.Environ(), "GO111MODULE=on")
 			cmd.Dir = m.Dir
 			cmd.Stderr = new(strings.Builder)