Preserve comments for Spack YAML objects (#11602)

This updates the configuration loading/dumping logic (now called
load_config/dump_config) in spack_yaml to preserve comments (by using
ruamel.yaml's RoundTripLoader). This has two effects:

* environment spack.yaml files expect to retain comments, which
  load_config now supports. By using load_config, users can now use the
  ':' override syntax that was previously unavailable for environment
  configs (but was available for other config files).

* config files now retain user comments by default (although in cases
  where Spack updates/overwrites config, the comments can still be
  removed).

Details:

* Subclasses `RoundTripLoader`/`RoundTripDumper` to parse yaml into
  ruamel's `CommentedMap` and analogous data structures

* Applies filename info directly to ruamel objects in cases where the
  updated loader returns those

* Copies management of sections in `SingleFileScope` from #10651 to allow
  overrides to occur

* Updates the loader/dumper to handle the processing of overrides by
  specifically checking for the `:` character
  * Possibly the most controversial aspect, but without that, the parsed
    objects have to be reconstructed (i.e. as was done in
    `mark_overrides`). It is possible that `mark_overrides` could remain
    and a deep copy will not cause problems, but IMO that's generally
    worth avoiding.
  * This is also possibly controversial because Spack YAML strings can
    include `:`. My reckoning is that this only occurs for version
    specifications, so it is safe to check for `endswith(':') and not
    ('@' in string)`
  * As a consequence, this PR ends up reserving spack yaml functions
    load_config/dump_config exclusively for the purpose of storing spack
    config