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 GNU Gettext.

* distro/packages/gettext.scm,
  distro/packages/patches/gettext-gets-undeclared.patch: New files.
* Makefile.am (MODULES): Add 'gettext.scm'.
  (dist_patch_DATA): Add 'gettext-gets-undeclared.patch'.