Provide an alternate design for work package relations

https://www.openproject.org/work_packages/12233

See also discussion:
* https://github.com/opf/openproject/commit/91ba7ba6fba688dc61b09215ae39813e3a79149f
* https://www.openproject.org/work_packages/11676
* https://github.com/opf/openproject/pull/1542

I might not be aware of the scope of this discussion, but I believe that `to` and `from` are good words when having a *directed* graph of work packages. So whereever both work packages are linked (e.g. in `/work_packages/{from_work_package_id}/relations/{to_work_package_id}` you will link to both WPs) this is apropriate wording.

In this specific case (relation as part of a "parent" WP), you could argue that the parent WP can be ommited and you will only need the related workpackage. However, in this case you should not introduce different vocabulary, but use the one that is already there. Thus (IMHO) `relatedWorkPackage` does not sound like the best choice.

*Note: Because I am not as involved as the two of you regarding this topic, I find it quite hard to follow some discussions and it is perfectly possible that I miss some very obvious facts . Don't be mad at me if I do, I just try to give helpful input.* :smiley_cat: