Fix handling of collect_ignore from parent conftest

`_collectfile` should be called on files only.

Fixes pytest-dev#4592.

src/_pytest/python.py

@@ -642,11 +642,12 @@ def collect(self):
             ):
                 continue
 
-            if path.isdir() and path.join("__init__.py").check(file=1):
-                pkg_prefixes.add(path)
-
-            for x in self._collectfile(path):
-                yield x
+            if path.isdir():
+                if path.join("__init__.py").check(file=1):
+                    pkg_prefixes.add(path)
+            else:
+                for x in self._collectfile(path):
+                    yield x
 
 
 def _get_xunit_setup_teardown(holder, attr_name, param_obj=None):

testing/test_collection.py

@@ -1144,3 +1144,16 @@ def test_nodeid(request):
         ]
     )
     assert result.ret == 0
+
+
+def test_collectignore_via_conftest(testdir, monkeypatch):
+    """collect_ignore in parent conftest skips importing child (issue #4592)."""
+    tests = testdir.mkpydir("tests")
+    tests.ensure("conftest.py").write("collect_ignore = ['ignore_me']")
+
+    ignore_me = tests.mkdir("ignore_me")
+    ignore_me.ensure("__init__.py")
+    ignore_me.ensure("conftest.py").write("assert 0, 'should_not_be_called'")
+
+    result = testdir.runpytest()
+    assert result.ret == EXIT_NOTESTSCOLLECTED