Prototype two-file "index plus updates" repodata format

[dholth]

Attempt to add libmamba support for https://github.com/conda/conda/blob/b8ef64e4ad27b36d39e519188203d0e2ff43925b/conda/gateways/repodata/jlap/README.md

Which collects patches into a second, smaller repodata.json overlay to avoid the problem of having to re-serialize the entire file all the time. If a package exists in the overlay, we skip loading it from the upstream repodata.

My C++ is rusty. I appreciate the C++ parser compared to what libsolv is doing. This does appear to be able to load additional packages from the overlay, from Python code.