hackage-db: Fix the type of signatures in metadata

[sternenseemann]

trafficstars

Previous PR/discussion: https://github.com/NixOS/hackage-db/pull/17

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Signatures are actually complex objects. We could use Value here since it's unlikely that any consumers care about this, but it's cheap enough to just define a structure for the signature objects.

In practice this isn't an issue because it seems like this field is never populated in Hackage, but it can be populated if you build a custom Hackage it can happen.

Fixes #573.

cc @michaelpj (sorry for the repository moving mess)

[michaelpj]

Thanks!