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/npm/package.py

@@ -36,7 +36,7 @@ class Npm(AutotoolsPackage):
     version('3.10.9', 'ec1eb22b466ce87cdd0b90182acce07f')
     version('3.10.5', '46002413f4a71de9b0da5b506bf1d992')
 
-    depends_on('node-js')
+    depends_on('node-js', type=('build', 'run'))
 
     def setup_dependent_environment(self, spack_env, run_env, dependent_spec):
         npm_config_cache_dir = "%s/npm-cache" % dependent_spec.prefix