_testcapimodule.c must not include pycore_pathconfig.h, since it's an
internal header files.

Changes:

* Add _PyCoreConfig_AsDict() function to coreconfig.c.
* Remove pycore_pathconfig.h include from _testcapimodule.h.
* pycore_pathconfig.h now requires Py_BUILD_CORE to be defined.
* _testcapimodule.c compilation now fails if it's built with
  Py_BUILD_CORE defined.