- 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
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