Better spack spec (#2238)

* Add options for hashes, tree depth, and YAML to `spack spec`.

- Can now display hashes with `spack spec`, like `spack find`.
  - Removed the old "ids" argument to `spack spec` (which
    printed numerical values)b

- Can output YAML spec from `spack spec` with `-y`

- Can control depth of DAG traversal with --cover=[nodes|edges|paths]

- Can print install status (installed, missing, not installed) with -I

* Don't use YAML aliases in specs.

- Force Spack's YAML dumper to ignore aliases.
- aliases cause non-canonical YAML to be used in DAG hash, and result in
  redundant hashes.
- add a test to ensure this behavior stays