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
