Cleanup: wasmer-registry

[theduke]

After the CLI work done in #4407 we now want to clean up the wasmer-registry crate.

Part of this work is tracked in #4408 .

Once that is done, we should:

• Move reusable generic logic to a different crate
• Move the CLI-internal things back into the CLI crate

Note that the code in wasmer-registry is really messy, untangling things won't be completely trivial.

[linear[bot]]

RUN-65 Cleanup: wasmer-registry