Pre-RFC: `pub(api)` visibility, and static enforcement of crate-internal API boundaries

I'm sure this has been discussed before. It would be cool to add an even more general and powerful feature instead.

vis api = vis!(in crate::resources);
vis anotherapi = vis!(in crate::resources);

pub(api) fn foo() {}
pub(anotherapi) fn bar() {}

This would completely solve the problem of

If we later decide to move the file_api module to a different location, we would have to update all of the visibility modifiers

And, I feel this gives developers a simpler model of visibility than needing to understand mod and #[api]. Adding another attribute #[api] doesn't feel like the most elegant solution here.

Granted, we are adding another keyword vis (bikeshedding allowed). And, we will need a syntax for vis!(). But, if we call my pre-pre-pre RFC idea here 'Vis Assignments', then it seems that Vis Assignments solve the same problems presented here but better. Happy to discuss. Vis Assignments would actually be pretty neat.