Revert language mixins back to module_function

[dduugg]

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See #15249 (comment)

cc @Bo98

[Bo98]

Happy to see this disappear the other way round (no module_function but also no self.) if you think it's worth it at some point (not in this PR since we need this to fix some homebrew-core CI).

[MikeMcQuaid]

Thanks @dduugg and @Bo98! This probably warrants:

• a rubocop at some point to disable/discourage additional module_function usage
• a comment on these modules so no-one comes along later and does this again

[dduugg]

Thanks @dduugg and @Bo98! This probably warrants:

• a rubocop at some point to disable/discourage additional module_function usage

I filed this in 2019, maybe I should actually build the thing…

• a comment on these modules so no-one comes along later and does this again

That, or tests that cover this case 😇