Move rustdoc-related code to rustdoc crate (#12945)

This PR moves the rustdoc-related code out of `html_to_markdown` and
into the `rustdoc` crate.

Release Notes:

- N/A

crates/rustdoc/src/crawler.rs

@@ -6,10 +6,10 @@ use async_trait::async_trait;
 use collections::{HashSet, VecDeque};
 use fs::Fs;
 use futures::AsyncReadExt;
-use html_to_markdown::convert_rustdoc_to_markdown;
-use html_to_markdown::structure::rustdoc::{RustdocItem, RustdocItemKind};
 use http::{AsyncBody, HttpClient, HttpClientWithUrl};
 
+use crate::{convert_rustdoc_to_markdown, RustdocItem, RustdocItemKind};
+
 #[derive(Debug, Clone, Copy)]
 pub enum RustdocSource {
     /// The docs were sourced from local `cargo doc` output.