Clean up python build before compiling library
Clean up python build before compiling library

When we rename or remove packages, the old setup.py build directory
can cause problems. This change deletes that directory before trying
to reinstall the library so that we don't get stale packages and
modules in the resulting Pegasus binary.

pegasus-isi/pegasus,pegasus-isi/pegasus,pegasus-isi/pegasus,pegasus-isi/pegasus,pegasus-isi/pegasus,pegasus-isi/pegasus,pegasus-isi/pegasus,pegasus-isi/pegasus,pegasus-isi/pegasus,pegasus-isi/pegasus