Previously we used nodePackages only to fetch Iosevka's build
dependencies and then fetched the source code ourselves. Updates
involved changing the version and hashes in the `iosevka` derivation and
then running node-packages/generate.sh to update the build dependencies,
which in turns updates *all* of node-packages.nix.
A new proposed policy for handling node-packages.nix updates involves
batching those updates. Previously, that would mean `iosevka` and its
build dependencies could end up out of sync until the batched update was
run.
To work with the new policy, we now fetch Iosevka's source code (and
not only its dependencies) through nodePackages. Updates are done by
changing the source URL in node-packages.json, which eventually becomes
part of an update of node-packages.nix, which is then propagated to
`iosevka` itself.
One con of this strategy is that errors can not be caught directly
after the update, but only after node-packages.nix is regenerated.
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