gnu: Consolidate databases in (gnu packages databases).
* gnu/packages/databases.scm: New file, with the contents of the
following deleted files.
* gnu/packages/mysql.scm, gnu/packages/postgresql.scm,
gnu/packages/recutils.scm, gnu/packages/sqlite.scm,
gnu/packages/bdb.scm: Remove.
* gnu-system.am (GNU_SYSTEM_MODULES): Adjust accordingly.
* gnu/packages/apl.scm, gnu/packages/dc.scm, gnu/packages/games.scm,
gnu/packages/gnunet.scm, gnu/packages/linux.scm, gnu/packages/mail.scm,
gnu/packages/mpd.scm, gnu/packages/nvi.scm, gnu/packages/openldap.scm,
gnu/packages/package-management.scm, gnu/packages/python.scm,
gnu/packages/qt.scm, gnu/packages/rdf.scm,
gnu/packages/version-control.scm: Adjust to new module name.