Ingest encourages use of invalid Items

[philvarner]

Currently, the ingest lambda requires that the Item collection field contains the name of the collection that the item will be ingested into. However, the STAC Item spec says that if collection is set, then there must be a link of rel=collection set, but the generator of the STAC Items that will be ingested does not necessarily know this URI, and in practice this is never set, but also it's stripped out when ingesting anyway.