[red-knot] symbol table should not own the flow graph

[carljm]

See #11624 (comment)

We probably do want to build symbol tables and flow graph (and find definitions) all in the same AST traversal, and we don't need to separate them since we either need all of them (if we're doing type inference within the module) or none of them (if we aren't). But it's weird for the symbol table to own the flow graph, as it currently does; probably both should be owned by some other structure instead, though I'm not sure what to name it. Index?