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-nbformat/package.py

@@ -35,7 +35,6 @@ class PyNbformat(PythonPackage):
     version('4.0.1', 'ab7172e517c9d561c0c01eef5631b4c8')
     version('4.0.0', '7cf61359fa4e9cf3ef5e969e2fcb909e')
 
-    depends_on('py-setuptools', type='build')
     depends_on('py-ipython-genutils', type=('build', 'run'))
     depends_on('py-traitlets', type=('build', 'run'))
     depends_on('py-jsonschema', type=('build', 'run'))