mdwsetup.py: Integrate better with `distutils'.

  * Move the file-generation functionality into new commands, and arrange
    to have them cleaned.

  * Override `sdist' to dereference symbolic links, rather than
    including them literally into tarballs (which breaks for out-of-tree
    links), and to write `RELEASE' into the tarball.

  * Add a command to extract the archive name, for release management.