Add PrefixData, SubdirData, and PackageCacheData to conda/api.py

[msarahan]

69f1b7a#diff-aa516f421733d7851d878e08d4bf7464R408

This is used in conda-build at https://github.com/conda/conda-build/blob/master/conda_build/render.py#L210

Please provide an interface (ideally in conda's exports) that I can use to ask Conda to download packages for me.

[kalefranz]

Conda 4.4 provides the Solver class in conda/api.py. In turn, that provides an instance of UnlinkLinkTransaction via the solve_for_transaction() method, which in turn provides an instance of ProgressiveFetchExtract via the get_pfe() method.

Perhaps that's progress. I don't think it's enough though.

In conda 4.5 we should target adding the PrefixData, SubdirData (representing repodata), and PackageCacheData classes to conda/api.py.

I'm still not sure that's enough.

I'd like to spend some time auditing all of conda-build's interactions with conda--with a lot of help from @msarahan--to see which pieces of conda-build could be dramatically simplified by preferring these interfaces over all of the legacy interfaces that are still being supported in conda.

[kalefranz]

I'm going to bump the milestone here to 4.5. I don't think it should block the 4.4.0 release.

[github-actions]

Hi there, thank you for your contribution to Conda!

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