[FEA] Split apart multi output conda recipe

[kkraus14]

Is your feature request related to a problem? Please describe.
As far as I can tell / dig up, there's no mechanism in conda-build to allow building only specific outputs of a recipe. With the changes introduced in #10326 I can no longer build just libcudf, but am forced to build the kafka integration library, tests, and example(s) unnecessarily.

Describe the solution you'd like
I'd like to be able to build a libcudf conda package via the conda recipe without having to build a handful of additional unnecessary things.

Describe alternatives you've considered
I can hack the unneeded pieces out of the conda recipe myself, but that's brittle and gross.

[jakirkham]

Thanks for the feedback Keith 🙏 Agree this is an important use case

Yeah there are probably ways we can influence this like adding environment variables or variants to optionally build subsets of packages

[github-actions]

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[vyasr]

Migrated to rapidsai/build-planning#61

[vyasr]

This will mostly be possible after rapidsai/build-planning#47 is completed, see rapidsai/build-planning#61 (comment) and prefix-dev/rattler-build#1425. The one caveat is that you will have to build the entire dependency tree of a given output, so you can't build libcudf-tests without building libcudf but you can build libcudf without building libcudf.tests. That seems like the right limitation to me though since otherwise you can't reliably satisfy a dependency (e.g. if we had pin_subpackage in place).