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

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.