distro: Go for one module per package.

The alternative, which was to use one module per category, would
probably not scale well.  First, because many packages could fall into
several categories (does GnuTLS go into "networking", "security", or
"libraries"?).  Second, because that could easily lead to circular
dependencies among modules ("security" and "networking" depend on each
other, etc.)

* distro/packages/databases.scm: Rename to...
* distro/packages/recutils.scm: ... this.
* distro/packages/typesetting.scm: Rename to...
* distro/packages/lout.scm: ... this.
* Makefile.am (MODULES): Adjust accordingly.
* po/POTFILES.in: Likewise.