New contract types: 'allow_graph' and 'forbid_graph'

[seddonym]

For more complex linting requirements that don't fit easily into the other built in types, we could have a contract that looks like this:

[importlinter:contract:my-graph-contract]
name = My contract
type = allow_graph
containers = mypackage
allow =
   blue, green -> yellow
   blue.alpha -> orange.beta

This contract would, for example, allow mypackage.blue.foo to import from modules contained in mypackage.yellow, but nothing else. Potentially, the expressions could have a richer syntax involving wildcards.

We could also have a forbid_graph type which list the things which aren't allowed instead.