[swig] Improve the native module import logic

The simple module import logic was not sufficient for our distribution
model of lldb, which is without the _lldb.pyd file (normally that would
be a symlink to the shared library, but symlinks are not really a thing
on windows).

With the older swigs it worked (loading of the python scripting
machinery from within lldb) because the normal swig import logic
contained a last-ditch import of a global module _lldb (which is defined
when you run python from lldb). Add back the last-ditch import to our
custom import logic as well.

llvm-svn: 305461