Consolidate most module code into spack.modules and spack.cmd.module

- One file with all the module classes (spack/modules.py)
  - Has an EnvModule superclass that does most of the work and consolidates common code
  - Subclasses have specializations for different module systems (TclModule, Dotkit)

- One command (spack module) for all the types of modules to use
  - the one command is used by the scripts, only need to maintain in one place
  - has some subcommands for different module types, but they're handled mostly generically.

- Consolidate zsh support into a single setup-env.sh script.

bin/spack

@@ -75,6 +75,13 @@ for cmd in spack.cmd.commands:
     module = spack.cmd.get_module(cmd)
     subparser = subparsers.add_parser(cmd, help=module.description)
     module.setup_parser(subparser)
+
+# Just print help and exit if run with no arguments at all
+if len(sys.argv) == 1:
+    parser.print_help()
+    sys.exit(1)
+
+# actually parse the args.
 args = parser.parse_args()
 
 # Set up environment based on args.