checkMapping takes kind absent, hybrid or draft. A draft has a name and one
or more prefixes and no books, and comes back with books []. A prefix inside
or holding another entry's book refuses library-playground-other-universe,
which replaces library-playground-outside-book. A prefix equal to a book or
holding its own refuses library-mapping-invalid, as does a draft source.
classify returns a playground file's entry and the book holding its prefix,
or null at universe level and in a draft.
jason.woltje