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/test_capture.py

@@ -15,7 +15,7 @@ from typing import TextIO
 import pytest
 from _pytest import capture
 from _pytest.capture import CaptureManager
-from _pytest.main import ExitCode
+from _pytest.config import ExitCode
 
 # note: py.io capture tests where copied from
 # pylib 1.4.20.dev2 (rev 13d9af95547e)