Deprecate module system, skip it for shaper modules

Now shaper is discovered via (previously unused!) font->find_shaper().
I'm keeping that just to allow clients override shaping.  Though,
even that I'm not sure we want to keep.

Wraps shaper in PangoEngineShape structs to keep PangoAnalysis API
intact.

Deprecated pango-modules.h and some pango-engine.h.  Language modules
are not moved yet.

Wired up PangoFc, PangoWin32, and PangoCoretext shapers.