Rollup merge of #84101 - jyn514:early-pass, r=Manishearth

rustdoc: Move crate loader to collect_intra_doc_links::early

This groups the similar code together, and also allows making most of collect_intra_doc_links private again.

This builds on https://github.com/rust-lang/rust/pull/84066, but it wouldn't be too hard to base it off master if you want this to land first.
Helps with https://github.com/rust-lang/rust/issues/83761.

r? manishearth

Fixes https://github.com/rust-lang/rust/issues/84046

src/test/rustdoc/intra-doc/auxiliary/empty.rs

@@ -0,0 +1 @@
+// intentionally empty

src/test/rustdoc/intra-doc/auxiliary/empty2.rs

@@ -0,0 +1 @@
+// intentionally empty

src/test/rustdoc/intra-doc/extern-crate-only-used-in-link.rs

@@ -1,8 +1,19 @@
+// This test is just a little cursed.
 // aux-build:issue-66159-1.rs
 // aux-crate:priv:issue_66159_1=issue-66159-1.rs
+// aux-build:empty.rs
+// aux-crate:priv:empty=empty.rs
+// aux-build:empty2.rs
+// aux-crate:priv:empty2=empty2.rs
 // build-aux-docs
-// compile-flags:-Z unstable-options
+// compile-flags:-Z unstable-options --edition 2018
 
 // @has extern_crate_only_used_in_link/index.html
 // @has - '//a[@href="../issue_66159_1/struct.Something.html"]' 'issue_66159_1::Something'
 //! [issue_66159_1::Something]
+
+// @has - '//a[@href="../empty/index.html"]' 'empty'
+//! [`empty`]
+
+// @has - '//a[@href="../empty2/index.html"]' 'empty2'
+//! [empty2<x>]