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/ns_path.py

@@ -0,0 +1,18 @@
+import sys
+import os
+from os.path import dirname
+
+sys.path.insert(0, os.path.join(dirname(__file__), 'namespace2'))
+sys.path.insert(0, os.path.join(dirname(__file__), 'namespace1'))
+
+#? ['mod1']
+import pkg1.pkg2.mod1
+
+#? ['mod2']
+import pkg1.pkg2.mod2
+
+#? ['mod1_name']
+pkg1.pkg2.mod1.mod1_name
+
+#? ['mod2_name']
+pkg1.pkg2.mod2.mod2_name