Rename rustdoc lints to be a tool lint instead of built-in.

- Rename `broken_intra_doc_links` to `rustdoc::broken_intra_doc_links`
- Ensure that the old lint names still work and give deprecation errors
- Register lints even when running doctests

  Otherwise, all `rustdoc::` lints would be ignored.

- Register all existing lints as removed

  This unfortunately doesn't work with `register_renamed` because tool
  lints have not yet been registered when rustc is running. For similar
  reasons, `check_backwards_compat` doesn't work either. Call
  `register_removed` directly instead.

- Fix fallout

  + Rustdoc lints for compiler/
  + Rustdoc lints for library/

Note that this does *not* suggest `rustdoc::broken_intra_doc_links` for
`rustdoc::intra_doc_link_resolution_failure`, since there was no time
when the latter was valid.

compiler/rustc_span/src/symbol.rs

@@ -1020,6 +1020,7 @@ symbols! {
         rustc_then_this_would_need,
         rustc_unsafe_specialization_marker,
         rustc_variance,
+        rustdoc,
         rustfmt,
         rvalue_static_promotion,
         sanitize,