config: split _getconftestmodules and _loadconftestmodules

Previously, the `_getconftestmodules` function was used both to load
conftest modules for a path (during `pytest_load_initial_conftests`),
and to retrieve conftest modules for a path (during hook dispatch and
for fetching `collect_ignore`). This made things muddy - it is usually
nicer to have clear separation between "command" and "query" functions,
when they occur in separate phases.

So split into "load" and "get".

Currently, `gethookproxy` still loads conftest itself. I hope to change
this in the future.

testing/python/fixtures.py

@@ -2103,9 +2103,7 @@ class TestAutouseManagement:
         reprec = pytester.inline_run("-v", "-s", "--confcutdir", pytester.path)
         reprec.assertoutcome(passed=8)
         config = reprec.getcalls("pytest_unconfigure")[0].config
-        values = config.pluginmanager._getconftestmodules(
-            p, importmode="prepend", rootpath=pytester.path
-        )[0].values
+        values = config.pluginmanager._getconftestmodules(p)[0].values
         assert values == ["fin_a1", "fin_a2", "fin_b1", "fin_b2"] * 2
 
     def test_scope_ordering(self, pytester: Pytester) -> None: