doc: Document basic package definitions.

* doc/guix.texi (Programming Interface): Add introduction.
  (Defining Packages): Populate.

Turn Guix into "GNU Guix".

* configure.ac: Change package name to "GNU Guix", and bug-report
  address to `gnu-system-discuss@gnu.org'.
* doc/guix.texi: Replace "Guix" by "GNU Guix" in some places.
  (Top, Introduction): Mention "for the GNU system".
* HACKING, README: Use "GNU Guix" instead of "Guix" in some places.

distro: Move bootstrap packages to (distro packages bootstrap).

* distro/packages/base.scm (glibc-dynamic-linker, %bootstrap-guile,
  bootstrap-origin, package-from-tarball, %bootstrap-base-url,
  %bootstrap-coreutils&co, %bootstrap-binutils, %bootstrap-glibc,
  %bootstrap-gcc, %bootstrap-inputs, package-with-bootstrap-guile): Move
  to ...
* distro/packages/bootstrap.scm: ... here.  New file.
* Makefile.am (MODULES): Add it.
* tests/builders.scm: Use (distro packages bootstrap).
  (%bootstrap-guile): Remove.
* tests/packages.scm: Likewise.
* tests/union.scm: Likewise, and remove @@ to access %bootstrap-inputs.
* tests/derivations.scm: Use (distro packages bootstrap) and remove @@
  to access %bootstrap-coreutils&co.
* HACKING (When the platform is supported by Nixpkgs): Update
  accordingly.

doc: Add "Adding new packages" in `HACKING'.

* HACKING (Adding new packages): New section.

doc: Add `HACKING'.

* HACKING: New file.
* Makefile.am (EXTRA_DIST): Add it.