Only call _collectfile on package instances

As discussed in #3751, this feels like a hack, pushing it only so we can
see how it fares on CI and if there are better solutions out there

src/_pytest/main.py

@@ -482,6 +482,8 @@ class Session(nodes.FSCollector):
             self.trace.root.indent -= 1
 
     def _collect(self, arg):
+        from _pytest.python import Package
+
         names = self._parsearg(arg)
         argpath = names.pop(0)
         paths = []
@@ -503,7 +505,7 @@ class Session(nodes.FSCollector):
                             root = self._node_cache[pkginit]
                         else:
                             col = root._collectfile(pkginit)
-                            if col:
+                            if col and isinstance(col, Package):
                                 root = col[0]
                                 self._node_cache[root.fspath] = root
 

testing/test_collection.py

@@ -647,7 +647,7 @@ class Test_getinitialnodes(object):
         col = testdir.getnode(config, x)
         assert isinstance(col, pytest.Module)
         assert col.name == "x.py"
-        assert col.parent.parent.parent is None
+        assert col.parent.parent is None
         for col in col.listchain():
             assert col.config is config