Add package dependency collection for Nimble packages

[BenJam]

Currently we just grab the list of packages from https://github.com/nim-lang/packages/blob/master/packages.json. Nimble supports git/hg sources so we'll then need to grab each git/hg repo in that list and parse its nimble file.

Requires #543

[dom96]

This is a bit off-topic but I didn't want to create a new issue for this: I'm wondering where the logo is specified for package managers. The current Nimble logo is outdated and it would be good to change it.

[andrew]

@dom96 updated it over here: librariesio/pictogram@899d4e0 will be deployed to the site later today, thanks for pointing that out