community/texmf-dist: provide subpackages at global scope

Don't extend $subpackages at runtime, but define it at global scope.

Sadly, this means manual effort to keep the subpackages in sync with the collections they pack. A check() function was added to check if for every collection a matching subpackage is defined. (If there is a subpackage without a corresponding collection, this will fail when packing anyway and is not explicitly tested.)