This reverts commit 9b90f68.

This reverts commit ef72e8b.

This solves the technical issue of upstream sources being unavailable. Which
doesn't block the build because the source is cached on cache.nixos.org but
breaks an aspect of reproducibility because others can no longer reproduce the
source without the binary cache.

As you can see, the hashes are all the same; no content has changed.

The implementation is quite complex because fetchgit isn't made to use mirrors
for submodules and submodules are also just kind of a pain to deal with in
general.

(The code could be deduplicated but it's expected to be obsolete in a few
month's time.)

yuzu-ea did not have to be touched; its source is still available.

Won't be getting updates any time soon :(