Use conda.base.constants.KNOWN_SUBDIRS for setting up selectors

[isuruf]

Description

Refactor conda_build.metadata.get_selectors to use conda.base.constants.KNOWN_SUBDIRS instead of duplicating this list in conda_build. Furthermore, abstract the selector generation so theres less effort to add new OS/Archs in the future.

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• Add / update outdated documentation?

[isuruf]

pre-commit.ci autofix

[JeanChristopheMorinPerso]

I see #5024. I had a little fear that we could loose some selectors by accident since the get_selectors function doesn't have any unit test.

I think the last thing would probably be to document that the supported subdirs come from conda. We might also want to go over https://docs.conda.io/projects/conda-build/en/stable/resources/define-metadata.html#preprocessing-selectors and update and add the supported selectors?

[jaimergp]

update and add the supported selectors?

Yes, I think that table is missing wasi, emscriptem and other recently added architectures. I wonder how tricky would be to autogenerate it like it's done in conda/conda for some parts.

[kenodegard]

@isuruf I've gone ahead and merged the latest changes from main into this branch since #5024 has been merged