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/python_moduleimport_runme.py

@@ -0,0 +1,7 @@
+import python_moduleimport
+
+if python_moduleimport.simple_function(99) != 99:
+    raise RuntimeError("simple_function")
+
+if python_moduleimport.extra_import_variable != "custom import of _python_moduleimport":
+    raise RuntimeError("custom import")