tools, loader, plugins.rules: deprecate compile_rule

[Exirel]

Description

See #1887.

This is only a draft, because it introduces a major change in loader.clean_callable: since compile_rule becomes deprecated, it is not used anymore by loader.clean_callable. It means that this function doesn't compile patterns anymore. So, even though it doesn't change anything for plugins that don't tinker with loader themselves (but why would they?), it doesn't generate any change and it should be transparent.

The documentation says that sopel.loader is for internal purpose, that it may and probably will change without warning, and that the documentation for this module is here for internal dev only. So, technically, it's not a breaking change, because that part of the API is documented as not public.

However, if we want to minimize the risk of a plugin relying on internal stuff (again: they should not), I've solutions to deal with that.

@dgw I need your opinion on that one!

Checklist

• I have read CONTRIBUTING.md
• I can and do license this contribution under the EFLv2
• No issues are reported by make qa (runs make quality and make test)
• I have tested the functionality of the things this change touches

[dgw]

I don't mind one bit changing how stuff in sopel.loader works, haha. There's just one random thing I noticed, but go ahead and change clean_callable() all you want! That whole module doesn't even appear in the docs.

[dgw]

I edited the OP to avoid closing #1887 (since it's a tracking issue, now assigned to 8.0), but let's do it.