Modify pylint/flake8 path helper to add lldb python module to path.

It was adding all the test infrastructure paths properly, but it
was not adding the lldb module.  The current approach only adds the
lldb from the path.  That can be improved (in the comments) to add
the one from the related build directory if it can be ascertained.

With this change, lldb tests can be run through pylint/flake8
and have the lldb module found and used as part of the checks.

llvm-svn: 250453