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.

build: Move `distro/patches' to `distro/packages/patches'.

* distro/patches: Move directory to...
* distro/packages/patches: ... here.
* Makefile.am (dist_patch_DATA): Adjust accordingly.
* pre-inst-env.in (DISTRO_PATCH_DIRECTORY): Likewise.

distro: Change relocatable-Guile patch to work around broken argv[0].

* distro/patches/guile-relocatable.patch: Change to use /proc/self/exe
  instead of PROGRAM_INVOCATION_NAME.  Useful because Nix itself passes
  just the basename of builders that it executes.

distro: Add a statically-linked, relocatable Guile 2.0 package.

* distro/packages/base.scm (%guile-static, %guile-static-stripped): New
  variables.

* distro/patches/guile-relocatable.patch: New file.
* Makefile.am (dist_patch_DATA): Add it.