LSIF: Ensure document URIs are always relative to dump root.
Created by: efritz
This PR makes an assumption explicit (one that we had before): an LSIF upload's projectRoot value will be either the repository root (which is now true in lsif-go and our fork of lsif-node via https://github.com/sourcegraph/lsif-go/pull/46 and https://github.com/sourcegraph/lsif-node/pull/10), or will be the path to the subproject being indexed.
No other projectRoot really makes sense, so I feel it's appropriate to enforce this on the indexer's side.