premis conversion should be checked when using buckets

Don't generate PiM for PREMIS which will not convert cleanly (specifically when using PiM buckets).

Example: A PREMIS event references a file (but nothing references the event). This event will not have an ADMID reference in the resulting PiM.

Start by tracing the Ruby conversion path used for PREMIS events and PiM buckets, then reproduce the example where an event references a file but nothing references the event. Done means the conversion no longer generates PiM for PREMIS that cannot convert cleanly, with the reported bucket case covered by a regression test.

Written by the indexing model from the issue text.