move "scriblib" from "racket-doc" to "scribble-lib"
count ".scrbl" files as modules within a package

The set of relevant extensions should be made extensible, eventually.
move Scribble tests to "scribble-test"
adjust the places <-> distributed-places connection

Instead of a dynamic-require, the connection goes through a
structure-type property on a `remote-note%' instance.