Auto merge of #23628 - emilio:path-shuffling-cleanup, r=Manishearth
Auto merge of #23628 - emilio:path-shuffling-cleanup, r=Manishearth

build: Cleanup some path-munging code.

I was looking at this today, and this seems better than the pre-existing code,
generally pre-pending to these paths isn't great...

<!-- Reviewable:start -->
---
This change is [<img src="https://reviewable.io/review_button.svg" height="34" align="absmiddle" alt="Reviewable"/>](https://reviewable.io/reviews/servo/servo/23628)
<!-- Reviewable:end -->

notriddle/servo,DominoTree/servo,notriddle/servo,KiChjang/servo,paulrouget/servo,nnethercote/servo,saneyuki/servo,larsbergstrom/servo,nnethercote/servo,KiChjang/servo,KiChjang/servo,saneyuki/servo,paulrouget/servo,paulrouget/servo,saneyuki/servo,nnethercote/servo,splav/servo,larsbergstrom/servo,splav/servo,notriddle/servo,larsbergstrom/servo,nnethercote/servo,KiChjang/servo,saneyuki/servo,DominoTree/servo,larsbergstrom/servo,splav/servo,DominoTree/servo,saneyuki/servo,DominoTree/servo,splav/servo,nnethercote/servo,splav/servo,larsbergstrom/servo,notriddle/servo,splav/servo,saneyuki/servo,KiChjang/servo,DominoTree/servo,larsbergstrom/servo,nnethercote/servo,larsbergstrom/servo,nnethercote/servo,larsbergstrom/servo,paulrouget/servo,saneyuki/servo,paulrouget/servo,DominoTree/servo,splav/servo,KiChjang/servo,paulrouget/servo,saneyuki/servo,paulrouget/servo,larsbergstrom/servo,saneyuki/servo,notriddle/servo,splav/servo,nnethercote/servo,splav/servo,larsbergstrom/servo,nnethercote/servo,KiChjang/servo,splav/servo,nnethercote/servo,paulrouget/servo,notriddle/servo,DominoTree/servo,notriddle/servo,notriddle/servo,DominoTree/servo,KiChjang/servo,paulrouget/servo,paulrouget/servo,DominoTree/servo,saneyuki/servo,KiChjang/servo,notriddle/servo,KiChjang/servo,DominoTree/servo,notriddle/servo