Eliminate `rustc_lint_defs`' dependency on `rustc_ast`.

It currently only depends on two things:
- `rustc_ast::AttrId`: this is just a re-export of `rustc_span::AttrId`,
  so we can import the original instead.
- `rustc_ast::AttributeExt`: needed only for the `name` and `id`
  methods. We can instead pass in the `name` and `id` directly.