module: Get rid of internals API

This removes gjs-internals-1.0.pc and gjs-module.h, and makes all the
module header files private, that were previously installed.

Removes "do not include separately" guards from those files, and makes
sure they are included with quotes instead of angle brackets. Untangles
any header inclusion problems resulting from the removal of gjs-module.h.

https://bugzilla.gnome.org/show_bug.cgi?id=772386