Auto merge of #10751 - blyxyas:explain_with_config, r=xFrednet

Add configuration options to `--explain`

This PR rearranges some modules, taking `metadata_collector` out of `internal_lints` and making public just the necessary functions for `explain()` to use.

The output looks something like this:
```sh
$ cargo run --bin cargo-clippy --manifest-path ../rust-clippy/Cargo.toml -- --explain cognitive_complexity
### What it does
Checks for methods with high cognitive complexity.

### Why is this bad?
Methods of high cognitive complexity tend to be hard to
both read and maintain. Also LLVM will tend to optimize small methods better.

### Known problems
Sometimes it's hard to find a way to reduce the
complexity.

### Example
You'll see it when you get the warning.

========================================
Configuration for clippy::cognitive_complexity:
- cognitive-complexity-threshold: The maximum cognitive complexity a function can have (default: 25)
```

Fixes #9990
r? `@xFrednet`

---

changelog: Docs: `cargo clippy --explain LINT` now shows possible configuration options for the explained lint
[#10751](https://github.com/rust-lang/rust-clippy/pull/10751)
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