Move most top-level lisp files to lib/lisp and lib/lisp/backend

Hopefully this should be neater in preparation for more refactorings
and cleanup.

* start-swank.lisp: Don't pass :LOAD-CONTRIBS to
SWANK-LOADER:INIT. In CREATE-SERVER, pass :DONT-CLOSE as T.

* swank-loader.lisp (src-files): Grok compound filename specs.
(*sysdep-files*): Change backend files into compound files specs
indicating subdirectories.

* swank.asd (defsystem :swank): Update file locations.

* lib/lisp/swank.lisp (sly-version-string): look for sly.el two
directories down.