git pkgs ecosystems

[andrew]

git pkgs info --ecosystems shows all ecosystems that Bibliothecary can parse, but there's no way to see which ecosystems actually have data in your database.

Add a command to list ecosystems present in the current repo's database, something like:

$ git pkgs ecosystems
rubygems: 232 dependencies
npm: 14 dependencies
actions: 8 dependencies

with --format=json option as well.