Lint to remove instances of `pub(self)` and `pub(in self)`

[LikeLakers2]

What it does

This lint would check for instances of pub(self) and pub(in self) and suggest that they be removed.

Advantage

Removes extraneous visibility syntax where it has no effect at all. Per Visibility and Privacy from the Rust Reference:

pub(self) [...] is equivalent to pub(in self) or not using pub at all.

Drawbacks

Some codebases may use pub(self) or pub(in self) to denote a item that is meant for use within a submodule, and the lack of such to denote an item meant only for use within the same file.

Example

pub(self) struct State;

Could be written as:

struct State;

[Centri3]

These two(three?) lints could probably be paired together. @rustbot claim

[LikeLakers2]

@Centri3 Fair - I just figure it's best to make two separate lint suggestion issues, since one's about code style and the other is about removing needless syntax.

[Centri3]

I meant they could be paired into the same PR, and lint pass :D separate issues is fine.