Support require one or the other nature

[laeubi]

Currently one can only require another nature by id, or one can only allow one or the other nature. What I recently came along (and seem not supported), is that one wants to require any of a set of natures (e.g. nature a or nature b) because both are exclusive (you can only have a or b on the project) but my nature wants at least one of them.

So from an extension point of view I then need to mark one as the "preferred" that is added if none of the ones I want are available.

[merks]

Could you provide an example of the context? Where/what is doing the requiring? Some nature requires another nature? Sorry to ask what might be a stupid question.

[laeubi]

https://github.com/eclipse-pde/eclipse.pde/blob/4a64a1140326cf5ba72eac305c2fb77ba3b83901/apitools/org.eclipse.pde.api.tools/plugin.xml#L123

Api tools requires org.eclipse.pde.PluginNature but with the new automatic manifest nature org.eclipse.pde.BndNature I want it to be able to require either plugin or bnd nature (but of course not both at the same time).