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``
bcb9226efb