Major refactoring
    
    This is a breaking change.  This commit doesn't update the test suite or the
    modlint tool, so even more so in that sense.  This will be addressed soon.
    
    To point out the changes:
    
    * the components section no longer includes packages subsections for component
      types; packages are listed directly under the component type identifier
    * components can now be marked with the buildorder tag
    * the validate() method was removed and property setters were enhanced instead
    * modules can now include other modules, for building stacks
    * the spec was cleaned up a bit
    * we're a lot less forgiving now; properties expect the exact types they work
      with and no in-place conversion is done for you
    * we're stricter when it comes to loading malformed modulemd files and we will
      go even further with upcoming commits
    
    The spec won't change much, if at all, before the 1.0 release.  The API may.
    
    Signed-off-by: Petr Šabata <contyk@redhat.com>