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.

tests/packages.scm

@@ -20,6 +20,7 @@
 (define-module (test-packages)
   #:use-module (guix store)
   #:use-module (guix utils)
+  #:use-module (guix hash)
   #:use-module (guix derivations)
   #:use-module (guix packages)
   #:use-module (guix build-system)
@@ -121,6 +122,66 @@
                                              (package-source package))))
     (string=? file source)))
 
+(test-equal "package-source-derivation, snippet"
+  "OK"
+  (let* ((file   (search-bootstrap-binary "guile-2.0.7.tar.xz"
+                                          (%current-system)))
+         (sha256 (call-with-input-file file port-sha256))
+         (fetch  (lambda* (store url hash-algo hash
+                           #:optional name #:key system)
+                   (pk 'fetch url hash-algo hash name system)
+                   (add-to-store store (basename url) #f "sha256" url)))
+         (source (bootstrap-origin
+                  (origin
+                    (method fetch)
+                    (uri file)
+                    (sha256 sha256)
+                    (patch-inputs
+                     `(("tar" ,%bootstrap-coreutils&co)
+                       ("xz" ,%bootstrap-coreutils&co)
+                       ("patch" ,%bootstrap-coreutils&co)))
+                    (patch-guile (package-derivation %store
+                                                     %bootstrap-guile))
+                    (modules '((guix build utils)))
+                    (imported-modules modules)
+                    (snippet '(begin
+                                ;; We end up in 'bin', because it's the first
+                                ;; directory, alphabetically.  Not a very good
+                                ;; example but hey.
+                                (chmod "." #o777)
+                                (symlink "guile" "guile-rocks")
+                                (copy-recursively "../share/guile/2.0/scripts"
+                                                  "scripts")
+
+                                ;; These variables must exist.
+                                (pk %build-inputs %outputs))))))
+         (package (package (inherit (dummy-package "with-snippet"))
+                    (source source)
+                    (build-system trivial-build-system)
+                    (inputs
+                     `(("tar" ,(search-bootstrap-binary "tar"
+                                                        (%current-system)))
+                       ("xz"  ,(search-bootstrap-binary "xz"
+                                                        (%current-system)))))
+                    (arguments
+                     `(#:guile ,%bootstrap-guile
+                       #:builder
+                       (let ((tar    (assoc-ref %build-inputs "tar"))
+                             (xz     (assoc-ref %build-inputs "xz"))
+                             (source (assoc-ref %build-inputs "source")))
+                         (and (zero? (system* tar "xvf" source
+                                              "--use-compress-program" xz))
+                              (string=? "guile" (readlink "bin/guile-rocks"))
+                              (file-exists? "bin/scripts/compile.scm")
+                              (let ((out (assoc-ref %outputs "out")))
+                                (call-with-output-file out
+                                  (lambda (p)
+                                    (display "OK" p))))))))))
+         (drv    (package-derivation %store package))
+         (out    (derivation->output-path drv)))
+    (and (build-derivations %store (list (pk 'snippet-drv drv)))
+         (call-with-input-file out get-string-all))))
+
 (test-assert "return value"
   (let ((drv (package-derivation %store (dummy-package "p"))))
     (and (derivation? drv)