[Docs] Generate the python reference without building all of LLDB

As discussed on the mailing list, we should be able to generate the
Python reference without building all of LLDB. To make that possible I
create a dummy python package, which is then parsed by epydoc. The
latter will complain that it couldn't import lldb, but that doesn't
matter as far as generation of the docs is concerned.

Differential revision: https://reviews.llvm.org/D61216

llvm-svn: 359465