Remove relative_fileloc

With the introduction of dag bundles, there's no longer a need for the fileloc / relative fileloc disctinction. _All_ dags are defined through bundles, so we can always just store the path relative to the bundle root.

Note that even if a bundle stores dags in a subfolder, the fileloc on the dag object should always be relative to the bundle root.

Start by tracing the fileloc and relative fileloc concepts through the DAG bundle implementation and their usages. Verify how DAGs in bundle subfolders are represented, then remove the distinction so every DAG path is relative to its bundle root. Done means the old distinction is no longer needed and the stated path rule holds consistently.

Written by the indexing model from the issue text.