Use `wasm-pkg-tools` to pull interface and world definitions from well-known registries

[`wasm-pkg-tools`](https://github.com/bytecodealliance/wasm-pkg-tools) provides tools for downloading published WIT definitions (in binary form) from registries, including [default registries](https://github.com/bytecodealliance/wasm-pkg-tools#default-fallback-registries). Ideally, if a `componentize-py` user targets a world that's been published to such a registry, e.g. `wasi:cli/command@0.2.0`, they shouldn't need to provide any WIT files (nor, in the case of WASI worlds, even specify the registry). Instead, `componentize-py` should use `wasm-pkg-tools` to query any available registries and download the world and interface definitions automatically.

In short, we should add a new option for specifying one or more registries (and use the default registries otherwise), plus another option to disable using the default registries if desired. I imagine we'll still want to prioritize using local WIT files if provided.