feat(pop): generate popularity on startup if url is not set

[maxhbr]

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[maxhbr]

Oh, you require a CLA,..

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[tazjin]

Thanks, this is a good idea! Two thoughts:

• I wonder if this should be on by default. Fetching all the required metadata is quite heavy, so it might be useful to make it opt-in instead.

• nixery-popcount is also a Go program, maybe its code should be used as a library instead in this use-case?