fix(build): scaffold plugins under plugins/<vendor>/<name> (quick-261028-pq8)

- make:plugin writes plugins/<vendor>/<name> with module
  <app>/plugins/<vendor>/<name>; the package name stays <name>
- make:* and the models-leaf check look in summer.yaml, then
  plugins/<vendor>/<name>, then the older plugins/<name>
- summer new puts its starter plugin at plugins/golem15/<name>
- refuse a vendor directory that is a legacy plugin module and a second
  copy of a plugin that already sits in plugins/<name>
- help text and docs, with an upgrading note: moving is optional

docs/examples/blog/scaffold_layout_test.go

@@ -46,7 +46,7 @@ func TestScaffoldLayout(t *testing.T) {
 		t.Fatalf("MakeCommand: %v", err)
 	}
 
-	scaffolded := fileSet(t, filepath.Join(app, "plugins", "blog"), func(rel string) bool {
+	scaffolded := fileSet(t, filepath.Join(app, "plugins", "acme", "blog"), func(rel string) bool {
 		return rel != "go.mod" && rel != "go.sum"
 	})
 	walkthrough := fileSet(t, ".", func(rel string) bool {