Support falling back to a global name lookup if a name isn't defined in the local scope, in the cases where that is correct according to Python semantics.

In class scopes, a name lookup checks the local namespace first, and if the name isn't found there, looks it up in globals.

In function scopes (and type parameter scopes, which are function-like), if a name has any definitions in the local scope, it is a local, and accessing it when none of those definitions have executed yet just results in an UnboundLocalError, it does not fall back to a global. If the name does not have any definitions in the local scope, then it is an implicit global.

Public symbol type lookups never include such a fall back. For example, if a name is not defined in a class scope, it is not available as a member on that class, even if a name lookup within the class scope would have fallen back to a global lookup.

This PR makes the @override lint rule work again.

Not yet included/supported in this PR:

• Support for free variables / closures: a free symbol in a nested function-like scope referring to a symbol in an outer function-like scope.
• Support for global and nonlocal statements, which force a symbol to be treated as global or nonlocal even if it has definitions in the local scope.
• Module-global lookups should fall back to builtins if the name isn't found in the module scope.

I would like to expose nicer APIs for the various kinds of symbols (explicit global, implicit global, free, etc), but this will also wait for a later PR, when more kinds of symbols are supported.