gokrazy: use monorepo for gokrazy appliance builds (monogok)

This switches our gokrazy builds to use a new variant of cmd/gok called
opinionated about using monorepos: https://github.com/bradfitz/monogok

And with that, we can get rid of all the go.mod files and builddir forests
under gokrazy/**.

Updates #13038
Updates gokrazy/gokrazy#361

Change-Id: I9f18fbe59b8792286abc1e563d686ea9472c622d
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

pkgdoc_test.go

@@ -71,6 +71,10 @@ func TestPackageDocs(t *testing.T) {
 			t.Logf("multiple files with package doc in %s: %q", dir, ff)
 		}
 		if len(ff) == 0 {
+			if strings.HasPrefix(dir, "gokrazy/") {
+				// Ignore gokrazy appliances. Their *.go file is only for deps.
+				continue
+			}
 			t.Errorf("no package doc in %s", dir)
 		}
 	}