- previously commands with this argument showed a long list of choices
  that were platform specific.

- use a better metavar: {defaults,system,site,user}[/PLATFORM]

- Add proper help for `spack buildcache` subcommands

- Reorganize the help categories of Spack commands so that buildcache is
  in packaging and diy and setup are now in build.

- Shorten Spack command usage for short options. Short options are now
  shown as [-abc] instead of as [-a] [-b] [-c]

- fix bug that mixed long and short options for top-level `spack help`

- command reference now includes usage for all Spack commands as output
  by `spack help`.  Each command usage links to any related section in
  the docs.

- added `spack commands` command which can list command names,
  subcommands, and generate RST docs for commands.

- added `llnl.util.argparsewriter`, which analyzes an argparse parser and
  calls hooks for description, usage, options, and subcommands