Add namespace fallback registry config

[lann]

In order to support a more natural registry resolution algorithm for package dependencies, add new configuration for "namespace fallback registries". The resolution algorithm would look something like:

For a parent package fetched from registry R, pick the first available:

• Local (non-"fallback") registry mapping
• Registry R's advertised namespace mapping (ref Add OCI namespace mappings to registry metadata (well-known URL) #35)
• Fallback registry mapping

Question: Should packages published by Registry R itself be considered before fallbacks?

[calvinrp]

Probably better that the namespace mappings were explicitly specified in the registry metadata well-known. Instead of attempting to fetch other non-specified namespaces from the OCI registry and hope that they are correct if available.

[lukewagner]

For the first bullet (local registry mapping), I could imagine it being useful to have a sub-option that lets specify that the local registry is just an overlay, in the sense that: if a package-name is present in the local registry, use that; otherwise fall back. This seems useful if, e.g., you're developing package ba:a and you need to patch a dependency ba:b, so you specify a local overlay for ba containing b, but everything else falls back to bytecodealliance.org.

[lann]

specify a local overlay for ba containing b, but everything else falls back to bytecodealliance.org

The config does already have "package overrides" which I think can serve this purpose.

[lukewagner]

Nice!