Allow the artifactIndex property to be absent when the artifactLocation is within an artifact

[lcartey]

Regarding the artifactIndex property of the artifactLocation object, the spec (section 3.4.5 on the eballot 3 draft) currently says:

If theRun.artifacts is absent or does not contain an element that describes the artifact specified by this artifactLocation object, then artifactIndex SHALL NOT be present.
...
Otherwise, artifactIndex SHOULD be present.

However, the artifactLocation is also used in the artifact object itself, where it would seem we should be able to omit the artifactIndex, as it is not required for lookup. In fact, the example in the artifactIndex section omits the artifactIndex in this way.

We could prohibit the use of artifactIndex in artifactLocations in artifact all together, but I think it might be useful to optionally allow it. This is because it would allow the use of the same in memory object for artifactLocations in both the artifact and places that reference the artifact.

[ghost]

@lukecartey Good catch. I added this sentence:

If thisObject occurs as the artifactLocation property (§3.23.2) of an artifact object in theRun.artifacts, then artifactIndex MAY be present. If present, it SHALL equal the array index within theRun.artifacts of the containing artifact object.

[ghost]

@lukecartey Actually, in addition to adding that sentence, I reworked the presentation of the entire logic of this section. It is logically the same but, I hope, clearer.