Convert conda-build entrypoints into plugins

[kenodegard]

Description

Might as well make conda-build into a proper plugin too!

This introduces a new submodule, conda.cli.plugins, where the pluggy hookimpls live. Otherwise minimal changes were made.

This also marks the legacy style plugin interfaces as deprecated, e.g.:

conda-build ...
python -m conda_build.cli.main_build ...

Resolves #4922

Checklist - did you ...

• Add a file to the news directory (using the template) for the next release's release notes?
• Add / update necessary tests?
• Add / update outdated documentation?

[jezdez]

Wait, this'll require a run constraint for a newer version of conda!

[kenodegard]

@jezdez added but technically not necessary? the old interface is still there and the plugin system is only imported in conda.plugin which is only invoked via the plugin system itself?

[travishathaway]

@kenodegard, I was testing this out locally today, and one thing I noticed is that the commands show up twice on the conda --help page. Once in the conda commands available from other packages: and another time in the ... (legacy).

Is there anything we can do to prevent that? I think that would require a change on the conda side.

[jezdez]

@kenodegard, I was testing this out locally today, and one thing I noticed is that the commands show up twice on the conda --help page. Once in the conda commands available from other packages: and another time in the ... (legacy).

Is there anything we can do to prevent that? I think that would require a change on the conda side.

I think we should stop shipping conda-* scripts?

[kenodegard]

conda-build's various subcommands will continue to be listed twice until conda is modified to exclude duplicates (probably the correct route?) or the executables are deprecated/removed

while the subcommand displaying twice is noisy its not incorrect (the legacy plugins are still there) and IMO isn't enough to block this PR