Auto merge of #66697 - petrochenkov:nocstore, r=eddyb
rustc_metadata: Privatize more things and a couple of other refactorings
This PR continues https://github.com/rust-lang/rust/pull/66496 and hits the point of diminishing returns.
All fields of `CrateRoot` and `CrateMetadata` are privatized.
For read-only fields this certainly makes sense, but for a few fields updateable from outside of `rmeta.rs` (mostly `creader.rs`) it was done mostly for consistency, I can make them `pub(crate)` again if requested.
`cstore.rs` (which became small after #66496) was merged into `creader.rs`.
A few things noticed while making the privacy changes were addressed in the remaining refactoring commits.