feat: make structure command elaborate as if structures are flat

[kmill]

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---

This PR <short changelog summary for feat/fix, see above>.

Closes <RFC or bug issue number fixed by this PR, if any>

[leanprover-community-bot]

Mathlib CI status (docs):

• 💥 Mathlib branch lean-pr-testing-7302 build failed against this PR. (2025-03-03 22:04:33) View Log