Rollup merge of #160924 - mejrs:split_cstore, r=JonathanBrouwer

split up `rustc_session`

Followup on https://github.com/rust-lang/rust/pull/160336 cc @JonathanBrouwer (feel free to review if you want)

The first commit splits out the `cstore` module from `rustc_session` into its own crate.

`rustc_session` actually never used it anywhere internally, and there are also several crates that only depend on `rustc_session` for it. So it is a natural candidate to split off.

The second commit is similar; `rustc_hir::definitions` is moved to `rustc_hir_id`; it is used nowhere in `rustc_hir`, except to implement an inherent method:

```rust
impl DefKind {
        pub fn def_path_data(self, name: Option<Symbol>) -> DefPathData { }
}
```

which is used (once) in rustc_middle, so the items in `rustc_hir::definitions` and this conversion function could be in many places. `rustc_hir_id` is the earliest and the easiest, so that's what I went with.

Together these allow `rustc_crate_store`, `rustc_hir` and `rustc_session` to compile in parallel.
Current graph:

<img width="825" height="617" alt="image" src="https://github.com/user-attachments/assets/aa954265-0057-4596-a7c4-38290937fb74" />

Graph after this pr:

<img width="853" height="572" alt="image" src="https://github.com/user-attachments/assets/9bcd16f8-897c-4990-920b-249500b2e78e" />