This is prepatory work for implementing jarmods support for
modpacks.ch, where we will need to look through files in a directory -
which would require that those files are present at such time.
This might even fix some weird bugs, maybe - I've not encountered any
bugs from how this previously worked, but I feel that what's going on
is slightly clearer now.
There aren't any (currently) packs on modpacks.ch that use Fabric, but
they do have support in their modpacklauncher. This patch just means
we preemptively have support should any packs that require it be
introduced.
