Extra dependencies conditioned by configuration values about get-deps

[pawamoy]

This syntax probably does not allow everything to be expressed, but we don't have a lot of extra dependencies conditionals like that in the ecosystem, so I think it's fine for now. But maybe we want to make the syntax super explicit and more expressive, to directly cater to more future uses:

extra_dependencies:
  # multiple deps supported with the same conditions
  - dependencies: [foo]
    conditions:
    - and:  # explicit AND, could be `or` too
      - extra.bar
      - extra.baz: [1, 2, 3]
      - extra.qux: [null, foo]