PathIndexBuilder builds un-necessary SegmentedIndices from EV links

[fredrikbk]

This was added in 6eac2fbd9c8157a924e1bbb864ec0d0807d5b3f1 to make EV matrices work, since we don't have codegen machinery to compute with elpack indices yet. When we get this machinery it should be changed back.

But even before then the PathIndexBuilder only needs to build a segmented index if it is going to be returned to the user. If it is just built internally to bootstrap a vev matrix then we can build an SetEndpointIndex instead.

Until then we do unnecessary work and consume unnecessary memory.