File paths in ChoCo partition-specific metadata to be improved

[jonnybluesman]

In each dataset partition, the metadata should always provide relative paths with respect to the root of the repository, rather than being relative to the location of the script that generated the metadata itself.

Example: ../../partitions/billboard/choco/jams/billboard_0.jams should be ./partitions/billboard/choco/jams/billboard_0.jams for consistency.