Improve asset directory to support generated directories

[silbinarywolf]

Whats changed?

We now have a build step called DirectoryFileInput which has a dependency on the directory given to it, we then make the aapt2link run command have a dependency on the DirectoryFileInput build step.

This means that:

• aapt2link will wait for DirectoryFileInput to complete
• This means DirectoryFileInput can resolve the lazy path of the directory and add the file inputs to aapt2link when the directory is "ready".

Changes

• Update asset and resources directory to automatically recursively add each file in a sub-folder as a file dependency to the run action.
• Fix SDL2 example being broken after previous version bump on Zig 0.15.2

Testing completed

• SDL2 example with Zig 0.15.2 on Linux via Android Studio
• Zig.bmp asset shows up in *.apk

Fixes #66

[silbinarywolf]

@Blayung Just letting you know this is more what I meant in this Github issue here: #62

Sorry if the reasoning for that was unclear or not obvious though in the original issue. Understanding the Zig build system stuff for myself took a while!

[Blayung]

@silbinarywolf Yeah, I had no idea you wanted an entire reusable build step, so I've only made a simple inline fix. I also don't know anything about generated directories. You're doing great work mate :D