PS: Add API graphs and models-as-data libraries

[MathiasVP]

This PR includes quite a few large changes to PowerShell:

1. It adds API graph functionality.
2. Then, it uses API graphs to implement models-as-data
3. Finally, it adds a bunch of local and remote flow sources. I've stolen these from the C# libraries, so we should be on par with C#'s .NET modelling in terms of flow sources.

There is still quite a few TODOs related to api graphs, so I don't expect this to work well everywhere yet. But it works for my local testing of flow sources.

Testing

I've not added any testing of this yet since I don't know what the best way to do this is. The problem is that, as I mentioned in #132, certain PowerShell modules are implicitly loaded from a path that's given by an environment variable which points to a set of manifest hash tables. So without extracting these manifest hash tables, we can't resolve those calls.

In #133 I disabled extraction of these hash tables in tests since we'd get spurious test changes depending on the environment in which the scripts were extracted. So we need to come up with a mock environment variable to extract "mock manifest files" for testing purposes. I've decided to delay this, though.