Move ExitCode's definition from _pytest.main to _pytest.config

ExitCode is used in several internal modules and hooks and so with type
annotations added, needs to be imported a lot.

_pytest.main, being the entry point, generally sits at the top of the
import tree.

So, it's not great to have ExitCode defined in _pytest.main, because it
will cause a lot of import cycles once type annotations are added (in
fact there is already one, which this change removes).

Move it to _pytest.config instead.

_pytest.main still imports ExitCode, so importing from there still
works, although external users should really be importing from `pytest`.

testing/python/collect.py

@@ -4,7 +4,7 @@ import textwrap
 
 import _pytest._code
 import pytest
-from _pytest.main import ExitCode
+from _pytest.config import ExitCode
 from _pytest.nodes import Collector