Refactor: Move semantic analyzer pass 3 to a new module (#4099)

Just move things around, tweak inter-module references and add
necessary imports.

The third pass still directly accesses the second pass so there is
more work to do.

Author: JukkaL

mypy/build.py

@@ -31,7 +31,8 @@
     from typing import Deque
 
 from mypy.nodes import (MypyFile, Node, ImportBase, Import, ImportFrom, ImportAll)
-from mypy.semanal import SemanticAnalyzerPass1, SemanticAnalyzerPass2, SemanticAnalyzerPass3
+from mypy.semanal import SemanticAnalyzerPass1, SemanticAnalyzerPass2
+from mypy.semanal_pass3 import SemanticAnalyzerPass3
 from mypy.checker import TypeChecker
 from mypy.indirection import TypeIndirectionVisitor
 from mypy.errors import Errors, CompileError, DecodeError, report_internal_error