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

internal/build/leaf.go

@@ -23,7 +23,11 @@ func checkModelsLeaf(appDir string, m Manifest) error {
 	for _, p := range m.Plugins {
 		pluginDir := localModuleDir(appDir, p.Module)
 		if pluginDir == "" {
-			pluginDir = filepath.Join(appDir, "plugins", pluginName(p.ID))
+			dir, err := defaultPluginDir(appDir, p.ID)
+			if err != nil || dir == "" {
+				continue
+			}
+			pluginDir = dir
 		}
 		if !underRoot(appDir, pluginDir) {
 			continue