Let a satellite module declare its endpoint kinds, so that its resources are generated and overridden the same way

Proposed resolution

• Let a module declare endpoint kinds of its own to the generator: the base class that does the work, the declarations its template writes, whether it is planned on every site or per bundle, and whether it is public by nature.
• Move the sitemap first: its logic into a base class of its module, its declaration generated with the same plugin ID and path, the handwritten class deleted.
• Plan a declared kind only while its module is installed, and take it out of the plan when the module goes.
• Coverage: a test module declares a kind; it is planned, written, answers, and a subclass takes it over.

Remaining tasks

Everything.