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_setuponly.py

@@ -1,5 +1,5 @@
 import pytest
-from _pytest.main import ExitCode
+from _pytest.config import ExitCode
 
 
 @pytest.fixture(params=["--setup-only", "--setup-plan", "--setup-show"], scope="module")