2803: Split arvados and arvados_fuse Python modules.

2803: Split arvados and arvados_fuse Python modules.

Prior commits tried to make separate SDK and FUSE packages from the
same sdk/python source tree.  However, this didn't work as intended
once the packages were installed.  Python modules don't work like Ruby
namespaces.  If you ask Python to import arvados.fuse, it finds the
arvados module, then looks for the fuse submodule under it.  So you
can't have arvados.fuse installed somewhere completely differently.

In addition, Python packages assume the installation script is called
setup.py.  So they wouldn't find setup_fuse.py.

We still want to have the FUSE module separate, so in discussion on
IRC we decided that the least worst option was to rename the module to
arvados_fuse.  This commit implements that.  If accepted, the new
build procedure will be:

  $ python setup.py egg_info -b \
    ".$(git log --format=format:%ct.%h -n1 .)" sdist upload

Refs #2803.