Avoid duplication of discovery for multi-sink sources

Perviously, where a single source has a pluggable discovery
extension loaded and is associated with multiple sinks, the
discovered resources were duplicated for each sink.

Now, we ensure that duplicate discovery extensions are
excluded.

Change-Id: Icffcc65f2be2baef7730b31773b39ffdda0454a7
Closes-Bug: #1354536
Merge "Avoid duplication of discovery for multi-sink sources"