feat(popcount): Add new narinfo-based popcount implementation

Adds an implementation of popcount that, instead of realising
derivations locally, just queries the cache's narinfo files.

The downside of this is that calculating popularity for arbitrary Nix
package sets is not possible with this implementation. The upside is
that calculating the popularity for an entire Nix channel can now be
done in ~10 seconds[0].

This fixes #65.

[0]: Assuming a /fast/ internet connection.

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