Issue description

I have several templates of my own, and while currently I keep them in ~/texmf, I would like to use Nix to build my reports and therefore cannot use that folder. How can I include these packages with texlive.combine?

The function texlive.combine combines derivations. However, the derivation needs to be more than just the contents of a texmf tree. Each TL package has also a passthru attribute pkgs and a tlType.

It would be nice if there would be a function that could be used for defining your own packages, and that can then be used with texlive.combine.

cc @vcunat