Use prepended modules instead of undef for OS-specific code

[dduugg]

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Relates to #17998

Switches from using undef, which results in (suppressed) sorbet errors, with prepended modules.

The latest proposal on the thread suggested creating an interface with three implementations, rather than using the generic-OS version in the default implementation, with methods undefined in OS-specific code. In building that out, I noticed that there wasn't a consistent, common interface among the OS variations, specifically in the case of Formula.

Thus, I've landed on OS extensions written as modules, which are then prepended to be used ahead of the generic implementations. (Note that in some cases, the modules are prepended to the singleton class, which admittedly gets pretty esoteric.)

[carlocab]

Nice

[MikeMcQuaid]

This approach looks good and makes sense to me, nice work!

I wonder if there's any way to avoid the need for the require at the bottom of the existing class? That'd be nice if possible.

[dduugg]

This approach looks good and makes sense to me, nice work!

I wonder if there's any way to avoid the need for the require at the bottom of the existing class? That'd be nice if possible.

Yes, that would be very nice, but right now sorbet/sorbet#5025 requires the prepend to come after the methods being replaced.

[MikeMcQuaid]

Thanks @dduugg! Still have some thoughts about the module naming/hierarchy but both the discussions and changes can 100% be done post-merge so no feedback here is blocking. Feel free to self-merge whenever you feel happy with it. Great work!