Implement comparison of #Target based on normalized path names.
Whether this relationship should use the same id as `other`.
Instances designating the same relationship need to use the same id. We can not simply compare the #Target attribute, though: `foo/bar.xml`, `../foo/bar.xml`, `../../foo/bar.xml` etc. are all different but probably mean the same file (this is especially an issue for relationships in the context of pivot tables). So lets just ignore this attribute for now (except when #TargetMode is set to `:External` – then #Target will be an absolute URL and thus can safely be compared).