Add wasm-pkg-common crate

[lann]

This PR includes:

• Common registry types:
  • Registry (domain)
  • PackageRef (namespace + name)
  • Label (lowercase kebab)
• Config, with package->registry mappings and individual registry configuration
  • Updates the syntax from warg-pkg-loader
  • Various methods to read and merge config from multiple sources
• RegistryMetadata, representing the content of /.well-known/wasm-pkg/registry.json
  • This updates the syntax, making the basic data structure more protocol-agnostic while maintaining (short-term) backward-compat

I want to solidify a config format as at least a medium-term compatibility guarantee; we can always introduce a config file versioning scheme for revisions later if needed, but its nice to avoid too much backward-compat code.

[calvinrp]

Just getting around to reviewing, but still going thru.

I think we can safely drop the backward compatible well-known logic / fields. We aren't relying on the behavior in our install instructions yet. And usage is very minimal for these tools. Can coordinate making the change.

[lann]

I think we can safely drop the backward compatible well-known logic / fields.

We can drop it in a future change; it just reduces the amount of coordination needed between various moving pieces.

[lann]

I was going to wait until I had some integration work done with this, but given that #29 depends on this now I'll just rebase what exists now for review.

[thomastaylor312]

Everything looks good to me