Add fallback registry support to wally search

[Synthetic-Dev]

Attempts to resolve #86

[Synthetic-Dev]

Testing using my own index.

Need to test:

• Circular fallback registries
• Fallback chains longer than 2
• Max depth flag for long chains

Possible addition:

• Mark packages with the same scope/name in lower fallback registries as unavailable?

[magnalite]

Great work! Sorry for taking so long to get back to this. PackageSourceMap already has logic for resolving the fallback chain which we can piggyback off here (PackageSourceMap.add_fallbacks()). The only issue is it presents this as a list of PackageSources. I suggest adding a method to PackageSource which allows us to send a request to the search backend of the registry and gives the raw response back. You'll want to ensure this branch is updated to the latest commit as PackageSource has been substantially redefined.

[Synthetic-Dev]

Not had time to work on this. Someone feel free to pick it up