3.4: Split module dependencies in two separete files

core-deps-base for "1st level" deps
core-deps      for "2nd and 3rd level" deps

https://mail.gnome.org/archives/desktop-devel-list/2011-May/msg00428.html

** 1st level **

  Established, stable, system modules, they have been in
  place for a long time, with stable API and ABI, and they exist in
  sufficient versions in the distributions commonly used by GNOME
  hackers, even in older but still used versions (Fedora 13 for
  example).

  Examples : libxml2, libpng, dbus...

  Proposed guideline : mentioned as dependencies with a base version,
  not built by default by jhbuild.

  Rationale : we want to reduce the number of modules that need to be
  built to start developing on GNOME.

** 2nd level **

  Modules developed outside GNOME, with little attention to our
  schedule, but with an active development, and where we want to track
  recent code.

  Examples : mozilla (js-185 nowadays), poppler.

  Proposed guideline : built from tarballs, version bumps whenever a
  module need a new version.

  Rationale : we need recent code, but we do not want to arrive on a
  release days with modules failing to build because they require some
  code only available in $DVCS.

** 3rd level **

  Modules developed outside GNOME, with attention to our schedule
  (i.e. we can ask for a tarball and get it in two days).

  Examples : webkitgtk, polkit.

  Proposed guideline : treated like any other GNOME module, built from
  latest git.

  Rationale : we do not need to put extra burden on modules that are
  close to us.