Firstly, this abstract away behavior in the mod download models that can
also be applied to other types of resources into a superclass, allowing
other resource types to be implemented without so much code duplication.
For that, this also generalizes the APIs used (currently, ModrinthAPI
and FlameAPI) to be able to make requests to other types of resources.
It also does a general cleanup of both of those. In particular, this
makes use of std::optional instead of invalid values for errors and,
well, optional values :p
This is a squash of some commits that were becoming too interlaced
together to be cleanly separated.
Signed-off-by: flow <flowlnlnln@gmail.com>
