The import resolution for namespace packages was wrong

With this change we can now include all parents of the script, which will make
relative imports always work.

Now the whole meta_path is scanned and not just importlib's PathFinder.

Fixes #1183.

test/examples/namespace_package_relative_import/rel1.py

@@ -0,0 +1 @@
+from .rel2 import name

test/examples/namespace_package_relative_import/rel2.py

@@ -0,0 +1 @@
+name = 1