An attribute that another attribute 'requires' should be allowed to be undefined if another attribute in the same set of 'requires' is an alternative

Right now ValidateModelStepHandler checks whether all attributes a defined attribute lists in 'requires' are themselves defined. It ignore the presence of alternatives settings. However in discussion with jfdenise_jira we've concluded that allowing undefined do to any 'alternatives' isn't right either. Rather, for a 'requires' to be superceded by an 'alternatives', only alternatives that are themselves part of the same set of 'requires' are relevant.

Imagine three scenarios:

I.
1) A requires B, C
2) B and C are alternatives to each other

User configures A and B is ok. Or, user configures A and C is ok. Configuring A, B, C is not ok.

II.
1) A requires B, C
2) B and C are alternatives to each other
3) B and D are also alternatives

User configures A and D and therefore must configure C, but not B. Configuring C satisfies the "requires" B.

III.
1) A requires B, C
2) B and C are NOT alternatives to each other
3) B and D are alternatives

User configures A therefore cannot configure D, must configure B and C. Configuring D does not satisfy the "requires" B.