packages: Add a 'snippet' field to <origin>.

packages: Add a 'snippet' field to <origin>.

* guix/packages.scm (<origin>): Add 'snippet', 'modules', and
  'imported-modules' fields.
  (patch-and-repack): Make 'inputs' a keyword parameter.  Add 'snippet',
  'modules', and 'imported-modules' parameters.  Accept SOURCE as a raw
  file name.  Insert SNIPPET in BUILDER.  Pass IMPORTED-MODULES to
  'build-expression->derivation'.
  (package-source-derivation): Pass the extra arguments to
  'patch-and-repack'.
* tests/packages.scm ("package-source-derivation, snippet"): New test.
* doc/guix.texi (Defining Packages): Mention the 'patches' and 'snippet'
  fields.
  (Invoking guix build): Tell that --source has patches and snippets
  applied.
  (Software Freedom): Mention packages that contain non-free code.