distro: Move patches to their own directory.

* Makefile.am (nobase_dist_guilemodule_DATA): Keep only $(MODULES).
  (patchdir): New variable.
  (dist_patch_DATA): New variable.  Patch files formerly
  in $(nobase_dist_guilemodule_DATA).
  (.scm.go): Define `DISTRO_PATCH_DIRECTORY' and
  `DISTRO_INSTALLED_PATCH_DIRECTORY'.
  (TESTS_ENVIRONMENT): Define `DISTRO_PATCH_DIRECTORY'.

* distro.scm (%patch-directory): New variable.
  (search-patch): New procedure.
* distro/base.scm: Use `search-patch' instead of `(search-path
  %load-path ...)'.

* distro/findutils-absolute-paths.patch, distro/guile-1.8-cpp-4.5.patch,
  distro/m4-readlink-EINVAL.patch, distro/m4-s_isdir.patch,
  distro/make-impure-dirs.patch: Move to `distro/patches'.