gnu: Python: bump to version 2.7.4.

* gnu/packages/python.scm: bump to version 2.7.4, remove references to
python-fix-dbm.patch.
* Makefile.am: remove references to python-fix-dbm.patch
* gnu/packages/patches/python-fix-dbm.patch: remove it.

gnu: Python: fix the compilation of some modules from the standard library.

This commit enables the bz2, dbm, readline, ssl and zlib modules.

* gnu/packages/gdbm.scm: Enable the compatibility mode.
* gnu/packages/python.scm: Enable a few modules from the standard library.
* gnu/packages/patches/python-fix-dbm.patch: New file.
* Makefile.am: Add it.

gnu: python: Remove unnecessary module import.

* gnu/packages/python.scm: Remove use of (gnu packages base).

distro: Change the module name space to (gnu ...).

* distro: Rename to...
* gnu: ... this.  Update module names accordingly.
* Makefile.am: Adjust accordingly.
* po/POTFILES.in: Likewise.
* distro.scm: Search for files under /gnu/packages instead of
  /distro/packages.
* gnu/packages/base.scm (ld-wrapper-boot3): Likewise.

distro: Add Python 2.7.3.

* distro/packages/python.scm: New file.
* Makefile.am (MODULES): Add it.