cosmo module core -> _core - #15126

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Taking myself off the reviewer list since I still dislike private modules...

Thinking more (mostly because of #15166, which affects utils), I think it would be good to have a more package-wide policy on how we indicate that submodules are private (in the sense that they should not be used to import from) -- see #15169. Note that I feel much more strongly about consistency than exactly what the plan is (and if cosmology deviates, perhaps that's OK too, but utils is another story).

Hi humans 👋 - this pull request hasn't had any new commits for approximately 4 months. I plan to close this in 30 days if the pull request doesn't have any new commits by then.

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I'm going to close this pull request as per my previous message. If you think what is being added/fixed here is still important, please remember to open an issue to keep track of it. Thanks!

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