[WIP] JExtractSwiftPlugin

[ktoso]

🚧 early work in progress 🚧

This is first steps to make an jextract-swift plugin.

If we're able to pull this off as a swiftpm plugin we would basically have a way to "make me a jar that exposes this swift module, that would be super simple to consume from Java.

Need to figure out how to actually make this work -- it can't be a build plugin, but preBuild also isn't quite enough.

We may have to make it a command plugin, since we need compilation OUTPUT of the module to then act on the swift interfaces.

[ktoso]

this can't work, since we need this to act on swiftinterface files of the module it's acting on. It has to be a "post build" plugin really.

[ktoso]

TODO: make this into a command plugin people would have to invoke manually swift package jextract which would generate the bindings.

We could also then swift package jextract jar to produce a jar or even publish it etc.

[ktoso]

Before weekend snapshot:

• we now generate swift thunks (!)
• first thunk is to "give me the Any.Type of this type" rather than using swift interfaces
  • these are done via public func swiftjava_getType_JExtractPluginSampleLib_MyCoolSwiftClass() -> Any /* Any.Type */ generated thunks
• jextract is now a prebuild plugin so we can get seamless build integration

Basically this means you "just" write some swift sources, add the extract plugin to the given target. Add a small task in gradle to trigger swiftpm and this way gradle/java immediately see the wrappers. Upon changing swift -> gradle triggers swift build again and it's all very seamless 🥳