PythonPackage builds flat installs instead of egg directories.

- Spack doesn't need eggs -- it manages its own directories

- Simplify install layout and reduce sys.path searches by installing all
  packages flat (eggs are deprecated for wheels, and this is also what
  wheels do).

- We now supply the --single-version-externally-managed argument to
  `setup.py install` for setuptools packages and setuptools.

- modify packages to only use setuptools args if setuptools is an
  immediate dependency

- Remove setuptools from packages that do not need it.

  - Some packages use setuptools *only* when certain args (likeb
    'develop' or 'bdist') are supplied to setup.py, and they specifically
    do not use setuptools for installation.

  - Spack never calls setup.py this way, so just removing the setuptools
    dependency works for these packages.

var/spack/repos/builtin/packages/py-ipywidgets/package.py

@@ -33,7 +33,6 @@ class PyIpywidgets(PythonPackage):
 
     version('5.2.2', '112f3daa4aa0f42f8dda831cea3649c8')
 
-    depends_on('py-setuptools', type='build')
     depends_on('python@2.7:2.7.999,3.3:')
     depends_on('py-ipython@4.0.0:', type=('build', 'run'))
     depends_on('py-ipykernel@4.2.2:', type=('build', 'run'))