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

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:

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.

Start by comparing each rustc_middle module with its similarly named crate, especially the MIR and rustc_mir distinction described in the issue. Record the purpose and boundaries of each pair, then add the findings to the rustc dev guide so the differences and possible split boundaries are clear.