When we hash the inputs to a MetaData node, we have to hash them in a
consistent order. We achieve this by sorting the stringfied `DefPath`
entries. Also, micro-optimie by cache more results across the saving
process.
For external crates, we must build up a map that goes from
the DefKey to the DefIndex. We do this by iterating over each
index that is found in the metadata and loading the associated
DefKey.
