Custom Python module importing code

Add optional moduleimport attribute to %module so that the
default module import code can be overridden. See the
"Searching for the wrapper module" documentation in Python.html.
Example:

  %module(moduleimport="import _foo") foo

$module also expands to the low-level C/C++ module name, so
the following is the same as above

  %module(moduleimport="import $module") foo

Issue https://github.com/swig/swig/issues/769

Examples/test-suite/python_moduleimport.i

@@ -0,0 +1,14 @@
+%define MODULEIMPORT
+"
+#print 'Loading low-level module $module'
+import $module
+#print 'Module has loaded'
+extra_import_variable = 'custom import of $module'
+"
+%enddef
+
+%module(moduleimport=MODULEIMPORT) python_moduleimport
+
+%inline %{
+int simple_function(int i) { return i; }
+%}