fix(import): support for nested namespace packages

If multiple directories in sys.path provide a nested namespace
package, then jedi would only visit the first directory which
contained the package.  Fix this by saving the remaining path list in
the ImplicitNamespaceValue and add a test for it.

test/completion/namespace1/pkg1/pkg2/mod1.py

@@ -0,0 +1 @@
+mod1_name = 'mod1'