Add dependents count fields on repositories stream

[laurentS]

This is WIP, as this commit requires changes in the SDK which are in a separate PR on that repo. Closes #34

@aaronsteers following our conversation on the issue, I've taken a stab at this.

• This relies on a MR on the SDK to add selected_by_default to Property. I thought setting this to false for the 2 extra fields would prevent users from getting this extra data unless they explicitly ask for it by setting selected: true in the catalog.
• The test I wrote will fail until the sdk supports this properly. I opened a merge request to help with fixing this, but it's WIP as well 🙈 as I would rather have your opinion before spending more time on it
• If things work how I expect them to, then this would avoid the need for a config option like I initially imagined, and I think this would be much cleaner and also reusable across taps.

[ericboucher]

I agree with @aaronsteers that adding this as a stream will make it easier and not require the MR on the SDk.

How about 3 streams @laurentS:

1. get dependents and dependencies counts
2. get dependents repos
3. get dependency repos

Have a look at their implementation:

• scrape-dependents: enable paging through package menu option if present dogsheep/github-to-sqlite#70