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.