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

[notriddle]

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

[rust-highfive]

r? @ollie27

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[jyn514]

Looking at this code more closely, it seems extremely fragile ... but that shouldn't block this immediate fix, which seems ok.

[jyn514]

@bors r+ p=1 (needs to land before Friday beta cutoff)

[bors]

📌 Commit 1e21dfa has been approved by jyn514

[bors]

⌛ Testing commit 1e21dfa with merge de4b242...

[bors]

☀️ Test successful - checks-actions
Approved by: jyn514
Pushing de4b242 to master...

[rust-timer]

Finished benchmarking commit (de4b242): comparison url.

Summary: This change led to very large relevant regressions 😿 in compiler performance.

• Very large regression in instruction counts (up to 6.2% on incr-unchanged builds of inflate)

If you disagree with this performance assessment, please file an issue in rust-lang/rustc-perf.

Next Steps: If you can justify the regressions found in this perf run, please indicate this with @rustbot label: +perf-regression-triaged along with sufficient written justification. If you cannot justify the regressions please open an issue or create a new PR that fixes the regressions, add a comment linking to the newly created issue or PR, and then add the perf-regression-triaged label to this PR.

@rustbot label: +perf-regression

[Mark-Simulacrum]

I cannot reproduce this regression locally, so it seems likely to be unfortunately spurious. Not clear what the cause might be (maybe incr-hashes dependent on some kind of environmental randomness?).