[typelib] Clean up dlopen handling

[typelib] Clean up dlopen handling

It's was busted that g_typelib_new_* does the dlopen() since that caused
g-ir-compiler to load the modules even though it wasn't going to do
anything with them.

Instead, change things so that g_module_symbol does the dlopen on-demand.
Remove the extra dlopen(NULL) inside girepository.c, we had another
already in gtypelib.c.

Thanks to Owen Taylor for suggesting this approach.