WASIX: Refactor Package Fetching and Module Caching

[theduke]

The wasix implementation currently supports fetching WebC packages and compiling them.
This functionality is exposed to guest code via proc_spawn , and via a virtual Wasmer command.

This currently is too deeply integrated into the code base, and should be refactored.

Fetching modules should be abstracted behind a ModuleResolver trait, which can be supplied/configured via the WasiRuntime trait.

The ModuleCache is too generic for the main wasix reference implementation, and should probably be removed in favour of more specialized implementations (including integrating with the default Wasmer cache).

NOTE: The refactor should also allow fetching packages from different registries, and not just rely on packages being namespace/name identifiers. Currently it's hardcoded to use the wapm.dev registry.

There already is a branch with a partial solution, but the changes were not included in the wasix branch to speed up the merge.