guix: Add lint-checker for packages which should be no inputs at all.

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.