Merge #7891

7891: Improve handling of rustc_private r=matklad a=DJMcNab

This PR changes how `rust-analyzer` handles `rustc_private`. In particular, packages now must opt-in to using `rustc_private` in `Cargo.toml`, by adding:
```toml
[package.metadata.rust-analyzer]
rustc_private=true
```

This means that depending on crates which also use `rustc_private` will be significantly improved, since their dependencies on the `rustc_private` crates will be resolved properly.

A similar approach could be used in #6714 to allow annotating that your package uses the `test` crate, although I have not yet handled that in this PR.

Additionally, we now only index the crates which are transitive dependencies of `rustc_driver` in the `rustcSource` directory. This should not cause any change in behaviour when using `rustcSource: "discover"`, as the source used then will only be a partial clone. However, if `rustcSource` pointing at a local checkout of rustc, this should significantly improve the memory usage and lower indexing time. This is because we avoids indexing all crates in `src/tools/`, which includes `rust-analyzer` itself.

Furthermore, we also prefer named dependencies over dependencies from `rustcSource`. This ensures that feature resolution for crates which are depended on by both `rustc` and your crate uses the correct set for analysing your crate.

See also [introductory zulip stream](https://rust-lang.zulipchat.com/#narrow/stream/185405-t-compiler.2Fwg-rls-2.2E0/topic/Fixed.20crate.20graphs.20and.20optional.20builtin.20crates/near/229086673)

I have tested this in [priroda](https://github.com/oli-obk/priroda/), and it provides a significant improvement to the development experience (once I give `miri` the required data in `Cargo.toml`)

Todo:
- [ ] Documentation

This is ready to review, and I will add documentation if this would be accepted (or if I get time to do so anyway)

Co-authored-by: Daniel McNab <[email protected]>

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