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-ipython/package.py

@@ -38,7 +38,6 @@ class PyIpython(PythonPackage):
     version('2.3.1', '2b7085525dac11190bfb45bb8ec8dcbf')
 
     depends_on('python@2.7:2.8,3.3:')
-    depends_on('py-setuptools@18.5:', type=('build', 'run'))
 
     # These dependencies breaks concretization
     # See https://github.com/LLNL/spack/issues/2793