use mol cache when tar archive missing

[tlitfin]

The current cache expects to see the mol.tar archive rather than the mol directory. This means that the download is triggered even if the mol directory is in the cache.

This is a small change to check for either mol or mol.tar before re-initiating the download.

[volgin]

boltz-community has a slightly more complete version of this fix — it checks for the extracted mols/ directory first, and only falls through to check for mols.tar if the directory is absent:

mols = cache / "mols"
tar_mols = cache / "mols.tar"
if not mols.exists():
    if not tar_mols.exists():
        # download mols.tar
    # extract mols.tar -> mols/

This handles both cases: mols/ already extracted (skip everything), and mols.tar present but not yet extracted (skip download, go straight to extraction). Might be worth adopting the same two-level check here.