Rename dep node "fingerprints" to distinguish key and value hashes

[Zalathar]

In the query system's dependency graph, each node is associated with two fingerprints: one that is typically a hash of the query key, and one that is typically a hash of the query's return value when called with that key.

Unfortunately, many identifiers and comments fail to clearly distinguish between these two kinds of fingerprint, which have very different roles in dependency tracking. This is a frequent source of confusion.

This PR therefore tries to establish a clear distinction between:

• Key fingerprints that help to uniquely identify a node (along with its DepKind), and are typically a hash of the query key
• Value fingerprints that help to determine whether a node can be marked green (despite having red dependencies), and are typically a hash of the query value

There should be no change to compiler behaviour.

r? nnethercote (or compiler)

[Zalathar]

• This is likely to have textual conflicts with Bring back enum DepKind. #152747.

[rust-bors]

☔ The latest upstream changes (presumably #152755) made this pull request unmergeable. Please resolve the merge conflicts.