Adds an optional query method to giomodules which should return all
possible extension points the module may implement.

Then we add a new call g_io_modules_scan_all_in_directory() similar to
g_io_modules_load_all_in_directory() that doesn't return all loaded
modules, thus allowing lazy loading.

In g_io_modules_scan_all_in_directory we look for an optional
giomodule.cache file and use the information in that to avoid
loading modules until they are needed for an extension point.