Replace conda_build/index.py with standalone conda-index

[dholth]

Checklist

• I added a descriptive title
• I searched open requests and couldn't find a duplicate

What is the idea?

conda-build generates repodata.json during its run, using get_build_index https://github.com/conda/conda-build/blob/main/conda_build/index.py#L112

This confusing function both downloads repodata.json, and generates new repodata.json for the local package cache / the packages that were just built.

It also provides the "conda-build index" command line interface, or "conda index".

Now that we have a much faster standalone conda-index descended from conda_build/index.py, conda-build should add a dependency on conda-index, and call that instead. (The downloading parts of get_build_index will probably have to stay)

• package conda-index
• figure out what to do about the new CLI "conda index" (provided by standalone conda-index) versus the old CLI "conda index" provided by conda-build
• depend on conda-index
• rewrite get_build_index

Why is this needed?

This will ensure that we have one excellent repodata.json generator.

What should happen?

No response

Additional Context

No response

[dholth]

Confusingly get_build_index is mostly about getting a list of available packages, not about creating an index, so we keep get_build_index and call conda_index's update_index() appropriately.

[dholth]

Tried again in #4798