Fix issue with relative import when using single header file import.

-- The commit propagates the package option to the newly create
     module node so it is recognized by SWIG
  -- Added a relativeimport test for this combination
     (in lack of py3 I was not able to test it with py3 but it
      "should just work")

Examples/python/import_packages/relativeimport3/README

@@ -0,0 +1,22 @@
+This example tests the %import directive and -relativeimport swig option.
+
+Use 'python runme.py' to run a test.
+
+Overview:
+---------
+
+The example defines 2 different extension modules--each wrapping a separate C++
+class.
+
+     pyX/pkg2/pkg3/foo.i   - Pkg3_Foo class
+     pyX/pkg2/bar.i        - Pkg2_Bar class derived from Pkg3_Foo
+
+The code is processed by swig with -relativeimport flag. The runtime test
+imports pyX.pkg2.bar module.
+
+If everything works well, the module pyX.pkg2.bar shall load properly.
+
+Unix:
+-----
+- Run make
+- Run the test as described above