feat: component bindgen: add support for multiple wit paths

[lwansbrough]

This change makes it possible to compile bindings from multiple wit source paths. It resolves #9249 and is heavily informed by the work done for wit-bindgen in bytecodealliance/wit-bindgen#1003

I had to modify one existing test (trappable_errors_with_versioned_and_unversioned_packages) to support this behaviour as the existing test was created without the expectation that the world in tests/codegen/unversioned-foo.wit would be considered a candidate world for selection.

[lwansbrough]

might need some guidance on how to setup the expanded tests

[alexcrichton]

Thanks for the work here! This all looks great to me, so all that's left is sorting out CI and tests and such. For the expanded tests you can run the tests with BINDGEN_TEST_BLESS=1 set in your environment to automatically update the expectations (and then check them in as a commit). For testing this feature itself feel free to use either the codegen test suite (which may mean you'll need to set up some directories if you don't use preexisting ones) or set up some directories in tests/all/component_model/*, whichever you find easiest.

[alexcrichton]

Thanks again for this!