Collapse all parent nested package fixtures when pointing to a sub-node.

Example:
Given this hierarchy: p1.s1.s2.s3
I want to run pytest p1/s1/s2/s3/foo.py

If there are any package fixtures defined at p1..s2 levels, they should also be executed.

_pytest/fixtures.py

@@ -75,10 +75,11 @@ def get_scope_package(node, fixturedef):
     cls = node.Package
     current = node
     fixture_package_name = os.path.join(fixturedef.baseid, '__init__.py')
-    while current and type(current) is not cls or \
-            fixture_package_name != current.nodeid:
+    while current and (type(current) is not cls or \
+            fixture_package_name != current.nodeid):
         current = current.parent
-    assert current
+    if current is None:
+        return node.session
     return current