rework spack help (#3033)

- Full help is now only generated lazily, when needed.
  - Executing specific commands doesn't require loading all of them.
  - All commands are only loaded if we need them for help.

- There is now short and long help:
  - short help (spack help) shows only basic spack options
  - long help (spack help -a) shows all spack options
  - Both divide help on commands into high-level sections

- Commands now specify attributes from which help is auto-generated:
  - description: used in help to describe the command.
  - section: help section
  - level: short or long

- Clean up command descriptions

- Add a `spack docs` command to open full documentation
  in the browser.

- move `spack doc` command to `spack pydoc` for clarity

- Add a `spack --spec` command to show documentation on 
  the spec syntax.

share/spack/qa/run-unit-tests

@@ -20,6 +20,10 @@ cd "$SPACK_ROOT"
 # Print compiler information
 spack config get compilers
 
+# Run spack help to cover command import
+${coverage_run} bin/spack -h
+${coverage_run} bin/spack help -a
+
 # Profile and print top 20 lines for a simple call to spack spec
 ${coverage_run} bin/spack -p --lines 20 spec mpileaks