[red-knot] refactor CFG outside of symbol table

[carljm]

Summary

Separate SymbolTable and FlowGraph into their own modules, and make both submodules of red_knot::semantic module. Also move types module into semantic.

red_knot::semantic::SemanticIndexer builds a SemanticIndex (consisting of the symbol table and flow graph). The symbol_table query is now semantic_index, and SymbolTablesStorage is SemanticIndexStorage.

Both SymbolTable and FlowGraph are immutable, and constructed using a separate builder.

Fixes #11657.

Test Plan

Refactor; existing tests pass.

[github-actions]

ruff-ecosystem results

Linter (stable)

✅ ecosystem check detected no linter changes.

Linter (preview)

✅ ecosystem check detected no linter changes.

[MichaReiser]

Thanks for doing this. I like this a lot. It makes the symbol module much more approachable.