Supports Textpacks.

Textpacks are moved to the textpattern/lang/ directory,
assigned under a provider.

Core Textpack for instance would be found from:
textpattern/lang/textpattern/textpacks/

While John's additional amendments would be located
in:
textpattern/lang/john/textpacks/

Now we need to update the language panel,
Textpattern setup script and build scripts and we are
all set.