linter(import/extensions): improve functionality

[wagenet]

This PR brings things more in line with the ESLint plugin's behavior here. However, we can't have full parity since the ESLint plugin relies upon file resolution. Where it did rely on this we try to err towards being permissive to avoid false positives.

This was primarily written with AI, though the code seems reasonable as far as I can tell.

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[codspeed-hq[bot]]

CodSpeed Performance Report

Merging #15009 will not alter performance

_{Comparing wagenet:better-import-extensions (c8509b7) with main (8c5b958)}

Summary

✅ 4 untouched
⏩ 33 skipped[^skipped]

[^skipped]: 33 benchmarks were skipped, so the baseline results were used instead. If they were deleted from the codebase, click here and archive them to remove them from the performance reports.

[connorshea]

See also #14602, I'm not sure which is the best to go with as the solution here but I definitely think we need to prioritize fixing this rule.

[connorshea]

Also this issue #12220, which I think this PR would solve?