Use & produce `run_exports.json` index file

[wolfv]

rattler-build could make use of the new run_exports.json which is a new file available "next" to the repodata.json file (e.g. https://conda.anaconda.org/conda-forge/osx-arm64/run_exports.json)

Then we don't need to download the packages to extract that information.

Our own indexing process should also produce this file.

And lastly, prefix.dev does not produce this file (yet) – but that's another issue.

[pavelzw]

Do quetz and artifactory produce these files?

[wolfv]

Quetz and prefix do not produce them yet. Would be nice to add the features, but we can always fall back.

No idea about artifactory.

It's a relatively new thing :)