Tweak lambda rule to use annotations rather than shadowing (#6044)
## Summary
This PR ensures that we can retain the current behavior even after we
reorder the visitor a bit, by looking for annotated lambdas rather than
"is the name bound to anything?", since if we visit the name before we
run this rule, it'll _always_ be bound. (This check is already a bit
flawed -- in truth, we should probably run this rule deferred so that we
can reliably detect shadowing.)
