New semantic analyzer: better error messages for cyclic cross-module definitions

[ilevkivskyi]

This is a follow-up for #6527

The new analyzer gives to verbose (and a bit confusing) error in this case:

[case testNewAnalyzerCyclicDefinitionCrossModule]
import b
[file a.py]
import b
x = b.x  # E: Cannot determine type of 'x'
[file b.py]
import a
x = a.x  # E: Cannot resolve attribute "x", possible cyclic definition \
         # E: Module has no attribute "x"

We can probably skip the second part by injecting a dummy Var with Any type in the a module namespace.