Previously, clippy (and any other tool emitting lints) had to have their
own separate UNKNOWN_LINTS pass, because the compiler assumed any tool
lint could be valid. Now, as long as any lint starting with the tool
prefix exists, the compiler will warn when an unknown lint is present.

It's not an internal lint:
- It's not in the rustc::internal lint group
- It's on unconditionally, because it actually lints `staged_api`, not
  the compiler

This fixes a bug where `#[deny(rustc::internal)]` would warn that
`rustc::internal` was an unknown lint.

This copies the unknown_lints code clippy uses for its
unknown_clippy_lints lint to rustc. The unknown_clippy_lints code is
more advanced, because it doesn't suggest renamed or removed lints and
correctly suggest lower casing lints.

This is now handled by unknown_lints