Also refactor some common code into a new function.
Examples for these pacakges are python(2)-setuptools and python(2)-pip, which
are installed together with python itself.
* guix/scripts/lint.scm (warn-if-package-has-input): New procedure.
(check-inputs-should-be-native package): Use it; rename and clean-up
variables. (check-inputs-should-not-be-an-input-at-all): New procedure.
(%checkers) Add it.
* doc/guix.texi (Python Modules): Document it.
* tests/lint.scm: ("inputs: python-setuptools should not be an input at all
(input)", "inputs: python-setuptools should not be an input at all
(native-input)" "inputs: python-setuptools should not be an input at all
(propagated-input)"): Add tests.
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