Found while working on test cases for SERVER-32308. Repro steps:

1. Create unsharded local and foreign collections, inserting a few documents to each.
2. Shard either collection through the first mongos, and ensure that the chunks are distributed across multiple shards.
3. Restart the primary shard to reset its metadata. It now believes that both collections are unsharded.
4. Send $lookup to a second mongos, which is unaware that either collection is sharded.