Rollup merge of #83216 - jyn514:register-tool, r=petrochenkov

Allow registering tool lints with `register_tool`

Previously, there was no way to add a custom tool prefix, even if the tool
itself had registered a lint:

 ```rust
 #![feature(register_tool)]
 #![register_tool(xyz)]
 #![warn(xyz::my_lint)]
 ```

```
$ rustc unknown-lint.rs  --crate-type lib
error[E0710]: an unknown tool name found in scoped lint: `xyz::my_lint`
 --> unknown-lint.rs:3:9
  |
3 | #![warn(xyz::my_lint)]
  |         ^^^
```

This allows opting-in to lints from other tools using `register_tool`.

cc https://github.com/rust-lang/rust/issues/66079#issuecomment-788589193, ``@chorman0773``
r? ``@petrochenkov``

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