Extract dependency WITs during build

[itowlson]

Fixes #3376.

WIP. Current status: 1. contains gross, gross hacks replaced with BEAUTIFUL AND ELEGANT hacks, and 2. incomplete.

Work needed:

• Handle export
• Test that include is handled correctly I think this works but a second tester would be very welcome
• Unit tests
• Provide a way to invoke it outside of build (can be deferred if naming is hard, which it always is)

[itowlson]

I think include might be a non-issue because Spin dependencies are on interfaces or packages, not on worlds, and interfaces don't include as far as I can tell (only worlds do).

I do determine the exported interfaces by looking at worlds, but I believe it's safe to ignore include directives for this, because an interface can be exported through an include if there's a world that exports it, and because we iterate the worlds we will pick it up from there.

I am not sure if there is some edge case when a component exports interfaces from other packages but that seems like a potential issue regardless of include. Bother.