Conditional redefinition of builtin doesn't work

[JukkaL]

Mypy complains about the type of next in the following example where we define a builtin if it doesn't exist:

try:
    next   # Error: Cannot determine type of next
except NameError:
    next = lambda x: x.next()

Mypy could fall back to the builtins module namespace during semantic analysis if it can't determine the type of a module-level name. This way the name would refer to the built-in definition until the first redefinition in the file.

(This is derived from #649, originally reported by @rbtcollins).