make "Package CONTAINS Package" valid even when files_analyzed == False

[armintaenzertng]

So far we assumed that all CONTAINS relationships point from a Package to a File. However, this is not specified by the spec, so a "Package CONTAINS Package" is also possible. This currently invalidates if files_analyzed is set to False for the first Package. This should be adapted to only invalidate in the "Package CONTAINS File" case.

Note that the relationship can also point to an SPDX element from an external document, in which case we can not determine whether the element is a File or Package or Snippet. In this case we have to assume that it is valid.