Use wasm-pkg-tools to pull interface and world definitions from well-known registries #220

wasm-pkg-tools provides tools for downloading published WIT definitions (in binary form) from registries, including default 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.