[racket] change to repository layout

For those who build from the Racket git repository but don't subscribe
to the dev mailing list: Beware that the repository layout is about to
change.

We are reorganizing the current Racket distribution into packages. When
the repository changes, the top level of the repository will contain
the following:

 * "racket" --- contains a pared-back version of the current
   repository, roughly.

   You could go into this directory, then into "src", and then use
   `configure', `make', and `make install' as usual. But you won't get
   DrRacket, or lots of other things. Instead, you'll get enough Racket
   to run `raco pkg' that could be used to install everything else.

   Even within this "racket" directory, there is some shuffling of
   files. Most notably, the "collects" directory has moved under "lib".

 * "pkgs" --- everything else in the current repository, but organized
   into packages.

   This organization is a first cut, and there will be lots of movement
   within this directory in the coming days and weeks.

 * "Makefile" --- a makefile to help you build Racket with all the
   usual packages.

   If you just type `make' in the top-level directory, you end up with
   a build in the "racket" subdirectory that includes DrRacket, etc.
   It builds the code in "racket" and then links in all the packages
   from "pkgs".

 * "README.txt" --- as you'd expect.

 * "INSTALL.txt" --- as you'd expect.

See "README.txt" and "INSTALL.txt" for more information, but there are
lots of detail that you can ignore. The main detail that you can't
ignore is that an in-place build ends up in a "racket" directory
instead of the immediate repository directory.

This first cut may work out, or we may change again, so expect further
changes. Please pardon our dust as we remodel.

Longer term, the "racket" directory may move to its own "racket-core"
repository, and the packages in "pkgs" may move to multiple other
repositories. If so, we expect that to happen months from now, not
within days or weeks.