* 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.
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