Auto merge of #7241 - flip1995:warn-deny-warnings, r=camsteffen

Deny warnings in every main sub-crate

Pointed out by `@xFrednet` in https://github.com/rust-lang/rust-clippy/pull/7229#issuecomment-842978909

This enables the same (rustc) lints in every main sub-crate:

- `clippy`
- `clippy_lints`
- `clippy_utils`
- `clippy_dev`

In addition it forwards the `deny-warnings` feature to those sub-crates, so we don't miss warnings that then become a problem during sync. (I wanted to fix that before, but forgot about it, so thanks for pointing it out `@xFrednet!)`

changelog: none