External command method backed workspaces.
BaseCmd class.

- For tracking an instance of external commands.
Tracking full, absolute paths instead.

- Relative paths may become a liability in the future when dealing with
  internal things split between multiple places.  Absolute paths make it
  very explicit.  However, to make it easier for users we may need to
  provide methods to return a list of relative paths instead.
Core command/binary calling methods

- An execute instancemethod for shortcut for the utility instance.
- An available classmethod for checking whether the class is available
  for use.
- Related private methods as shortcuts for the alternative usages.