distro: Change $DISTRO_{PATCH,BOOTSTRAP}_DIRECTORY to search paths.

* distro.scm (not-colon): New variable.
  (%patch-directory): Rename to...
  (%patch-path): ... this.  Turn into a list.  Expect $DISTRO_PATCH_PATH
  to be a colon-separated search path.
  (%bootstrap-binaries-directory): Rename to...
  (%bootstrap-binaries-path): ... this.  Likewise.
  (search-patch, search-bootstrap-binary): Adjust accordingly.

* pre-inst-env.in: Change to use `DISTRO_PATCH_PATH' and
  `DISTRO_BOOTSTRAP_PATH'.

distro: Use our own pre-built Guile to bootstrap.

* distro/packages/base.scm (%bootstrap-guile): Build from a tarball
  containing a pre-built Guile.

* distro.scm (%bootstrap-binaries-directory): New variable.
  (search-bootstrap-binary): New procedure.

* Makefile.am (bootstrapdir, bootstrap_x86_64_linuxdir,
  dist_bootstrap_x86_64_linux_DATA, DISTCLEANFILES, DOWNLOAD_FILE): New
  variables.
  (distro/packages/bootstrap/x86_64-linux/guile-bootstrap-2.0.6.tar.xz):
  New rule.
  (EXTRA_DIST): Add `build-aux/download.scm'.
  (.scm.go): Define the `DISTRO_BOOTSTRAP_DIRECTORY' and
  `DISTRO_INSTALLED_BOOTSTRAP_DIRECTORY' environment variables.

* pre-inst-env.in: Define `DISTRO_BOOTSTRAP_DIRECTORY'.

* build-aux/download.scm: New file.
* distro/packages/bootstrap/x86_64-linux/{bash, mkdir, tar, xz}: New
  files.

distro: Fix typo in warning message.

* distro.scm (package-files): Add missing newline in warning message.

distro: Add the `guile', `typesetting', and `databases' package modules.

* distro.scm (package-files): Adjust PREFIX-LEN to use the top-level
  directory, not %DISTRO-MODULE-DIRECTORY.
* distro/packages/base.scm (guile-reader, guile-reader/guile-1.8,
  guile-reader/guile-2.0, lout, recutils): Move to...
* distro/packages/guile.scm, distro/packages/typesetting.scm,
  distro/packages/databases.scm: ... here.  New files.
* Makefile.am (MODULES): Add them.
  (EXTRA_DIST): Add `.dir-locals.el'.

* .dir-locals.el: New file, with settings formerly in `base.scm'.

distro: Rename (distro ...) to (distro packages ...).

* distro/base.scm, distro/ld-wrapper.scm: Move to `distro/packages'.
  Adjust LD-WRAPPER-BOOT3 input file name accordingly.
* Makefile.am (MODULES): Adjust accordingly.
* distro.scm (%distro-module-directory): Change to "/distro/packages".
* guix/build-system/gnu.scm (standard-inputs): Change module name
  to (distro packages base).
* tests/packages.scm (test-packages): Likewise.

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'.

Add (distro) with a package searching facility.

* Makefile.am (MODULES): Add `distro.scm'.

* distro.scm: New file.
* po/POTFILES.in: Add it.

* tests/packages.scm ("find-packages-by-name", "find-packages-by-name
  with version"): New tests.