fix(doctest): detect extern crate items in statement doctests

This partially reverts #91026, because rustdoc needs to detect the extern statements,
even when they appear inside implicit `main()`. It does not entirely revert it,
so the old bug is still fixed, by duplicating some of the logic from `parse_mod`
instead of trying to use it directly.

Fixes #91134

src/test/rustdoc-ui/auxiliary/empty-fn.rs

@@ -0,0 +1,3 @@
+// no-prefer-dynamic
+#![crate_type = "lib"]
+pub fn empty() {}

src/test/rustdoc-ui/issue-91134.rs

@@ -0,0 +1,14 @@
+// compile-flags: --test --crate-name=empty_fn --extern=empty_fn --test-args=--test-threads=1
+// aux-build:empty-fn.rs
+// check-pass
+// normalize-stdout-test: "src/test/rustdoc-ui" -> "$$DIR"
+// normalize-stdout-test "finished in \d+\.\d+s" -> "finished in $$TIME"
+// edition:2021
+
+/// <https://github.com/rust-lang/rust/issues/91134>
+///
+/// ```
+/// extern crate empty_fn;
+/// empty_fn::empty();
+/// ```
+pub struct Something;

src/test/rustdoc-ui/issue-91134.stdout

@@ -0,0 +1,6 @@
+
+running 1 test
+test $DIR/issue-91134.rs - something (line 10) ... ok
+
+test result: ok. 1 passed; 0 failed; 0 ignored; 0 measured; 0 filtered out; finished in $TIME
+