Add documentation and tests for setup_helpers.pkg_config

The ``setup_helpers.pkg_config`` function is used in several of astropy's ``setup_package.py`` files, so we should keep it here, but it isn't tested or documented, so that will need to be fixed.

Locate setup_helpers.pkg_config and the astropy setup_package.py files that use it. Read the function's current behavior first, then add documentation and tests that cover its supported use; done means the function is documented and its behavior is exercised by the test suite.

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