Explain difference between `rustc_middle::{something}` and `rustc_{something}` in rustc dev guide

[camelid]

I think we are all agreed that splitting up rustc_middle would be a good idea
as it would reduce memory usage and increase parallelism when bootstrapping.
Ideally, we would split out each rustc_middle module into its own crate. For
example, rustc_middle::ich would become rustc_ich or something (I'm working
on this right now).

However, a lot of the rustc_middle modules already have crates named after
them:

rustc_middle module	similarly-named crate
rustc_middle::arena	rustc_arena
rustc_middle:hir	rustc_hir
rustc_middle::infer	rustc_infer
rustc_middle::lint	rustc_lint
rustc_middle::macros	rustc_macros
rustc_middle::mir	rustc_mir
rustc_middle::query	rustc_query_system
rustc_middle::traits	rustc_traits
rustc_middle::ty	rustc_ty

I believe for the case of rustc_middle::mir -> rustc_mir the difference is
that rustc_middle::mir contains the definition for the MIR and associated code
like the MIR visitors, while rustc_mir contains analysis code (e.g. borrowck),
but I don't know what the differences are for the others. Therefore, my goal
with this meta issue is to collect information about what the differences
are to make it easier to split up rustc_middle.

[camelid]

See also rust-lang/rust#65031.

[Mark-Simulacrum]

The difference -- in large parts -- is "pieces which (probably) interact with other parts of rustc_middle" -- I don't think the distinction is meaningful beyond that. It is also somewhat unclear to me what you expect from this issue to be honest -- responses from various team members? :)

[camelid]

Yes, I was hoping for different team members to reply, but I wasn't sure whether to ping people, so I posted on Zulip.

[estebank]

It is also somewhat unclear to me what you expect from this issue to be honest -- responses from various team members?

It could result in expanding the dev guide with our collective wisdom :)

[Enselic]

It makes sense to move this to the rustc-dev-guide repo then, right?

@rustbot transfer rustc-dev-guide