[red-knot] infer_symbol_public_type infers union of all definitions

[carljm]

Summary

Rename infer_symbol_type to infer_symbol_public_type, and allow it to work on symbols with more than one definition. For now, use the most cautious/sound inference, which is the union of all definitions. We can prune this union more in future by eliminating definitions if we can show that they can't be visible (this requires both that the symbol is definitely later reassigned, and that there is no intervening call/import that might be able to see the over-written definition).

Test Plan

Added a test showing inference of union from multiple definitions.

[codspeed-hq]

CodSpeed Performance Report

Merging #11667 will improve performances by 11.01%

_{Comparing carljm:cfg (18a64d8) with main (b80bf22)}

Summary

⚡ 1 improvements
✅ 29 untouched benchmarks

Benchmarks breakdown

	Benchmark	main	carljm:cfg	Change
⚡	parser[numpy/ctypeslib.py]	1.2 ms	1 ms	+11.01%

[github-actions]

ruff-ecosystem results

Linter (stable)

✅ ecosystem check detected no linter changes.

Linter (preview)

✅ ecosystem check detected no linter changes.

[carljm]

Closing in favor of #11669, which uses a source branch in the Astral repo instead of my own repo; this lets me stack PRs.