Add configuration options to --explain

[blyxyas]

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:

$ 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

[xFrednet]

I've only skimmed over the code, but we might be able, to only move ClippyConfiguration outside of the internal feature. That would make those dependencies still optional. Would that be an option @blyxyas

Btw, thank you for giving this a shot, the idea already looks good ❤️

[blyxyas]

Ok, the new commit minimizes changes and build times (53.15s from 58s). Keeping the same dependencies.
It also changes the configuration section to a more Markdown-like style:

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

[flip1995]

I also really like the idea! Thanks for keeping those deps optional and measuring the compile time and sizes. I'll now leave this PR to you and the friendly penguin to review :)

[xFrednet]

I'm happy that you kept the doc comments ❤️

[xFrednet]

I would suggest moving this into format into a method on the configuration struct, to have it more uniform with the other formatting code. However, that is just a style question, and I can see reasons for having it here, so this is an optional suggestion :)

---

I feel like the formatting is a bit hard to read, for lints with multiple configs, like in this example:

> cargo run --bin cargo-clippy --manifest-path ../rust-clippy/Cargo.toml -- --explain manual_let_else


### Configuration for clippy::manual_let_else:
    - msrv: The minimum rust version that the project supports (default: None)
    - matches-for-let-else: Whether the matches should be considered by the lint, and whether there should
    be filtering for common types. (default: WellKnownTypes)

But I also don't know how to improve it much. It should be fine, since we have very few lints with multiple config values :)

[blyxyas]

About the first issue: I'll fix it.
About the second: I have 0 ideas about how to fix this. Maybe adding a newline between ### Configuration for ... and the options?

[blyxyas]

After taking a look at the code again, I don't think I can even fix the first problem.
Each configuration option depends on mdconf and config_vec_positions. While mdconf is independent (taken from get_configuration_metadata()), config_vec_positions depends on info.

I think extrapolating the whole fragment (from line 480 to 491) would abstract so much the process of getting the configuration, that it would difficult future contributors and would make the function more readable, but less understandable overall.

I'll still fix the formatting.

[xFrednet]

Alright, then just keep it as is. Fixing the indention was a nice catch! Thank you, I'll leave you then honer of ordering @bors to merge it :D

[blyxyas]

omg thx ❤️

[xFrednet]

Looks good to me, I have one small optional NIT, and then it's ready to be merged. You can r=me with or without the suggestion. (r=me meaning commenting bors r=xFrednet with an @ in front)

@bors delegate+

[xFrednet]

@bors delegate+

[bors]

✌️ @blyxyas can now approve this pull request

[blyxyas]

@bors r=xFrednet

[bors]

📌 Commit 3e8fea6 has been approved by xFrednet

It is now in the queue for this repository.

[bors]

⌛ Testing commit 3e8fea6 with merge 1407c76...

[bors]

☀️ Test successful - checks-action_dev_test, checks-action_remark_test, checks-action_test
Approved by: xFrednet
Pushing 1407c76 to master...