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/services/storage.md

@@ -49,7 +49,7 @@ module: git.golem15.com/acme/acme
 binary: acme
 plugins:
   - id: acme.blog
-    module: git.golem15.com/acme/acme/plugins/blog
+    module: git.golem15.com/acme/acme/plugins/acme/blog
 build:
   tags:
     - summercms_s3