cmd/doc: treat any non-empty GOMOD as module mode

Previously, we were looking for the string go.mod specifically, but
the module-mode-outside-a-module logic added in CL 148517 sets GOMOD
to os.DevNull

Updates #28992

Change-Id: I62a4baaa911a495350294d78bae96be3fe4866cb
Reviewed-on: https://go-review.googlesource.com/c/151617
Run-TryBot: Bryan C. Mills <bcmills@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Russ Cox <rsc@golang.org>

src/cmd/doc/dirs.go

@@ -162,7 +162,7 @@ func findCodeRoots() []Dir {
 		// Check for use of modules by 'go env GOMOD',
 		// which reports a go.mod file path if modules are enabled.
 		stdout, _ := exec.Command("go", "env", "GOMOD").Output()
-		usingModules = bytes.Contains(stdout, []byte("go.mod"))
+		usingModules = len(bytes.TrimSpace(stdout)) > 0
 	}
 
 	if !usingModules {