Add support for MKCL

* swank-mkcl.lisp: New file. Mostly the one from MLCL's contrib/
directory.
* swank-loader.lisp (*sysdep-files*): Add swank-mkcl and move
swank-gray to the shared *swank-files*.
Move backend files to new swank/ directory.

* swank-loader.lisp (src-files): Treat lists like (swank foo) as
the filename swank/foo.lisp.
In the swank/ directory drop the swank- prefix from filenames.
Use swank/ as prefix in some packages.

To mirror the directory structure in package names use swank/
instead of swank- as prefix.  (This will presumably break a lot of
code as many people seem to assume that a swank-backend package
exists.)