Use `wasi-virt`-based VFS to store Python code

Currently, `componentize-py` "cheats" by assuming all required Python code will be loaded during the component pre-init step, and thus CPython won't need access to any of those files at runtime. This works surprisingly well, but will certainly break for more dynamic apps.

We should probably build a VFS by default, but also offer an option to use the "cheat" if desired.