incr.comp.: Use DefPathHash-based DepNodes in the serialized DepGraph and remove obsolete DefIdDirectory
With this PR we don't store the dep-graph as a set of `DepNode<IndexIntoDefIdDirectory>` anymore but instead as a set of `DepNode<DefPathHash>`. Since a `DefPathHash` is a global identifier that is valid across compilation sessions, we don't need the `DefIdDirectory` anymore.
Since a `DepNode<DefPathHash>` is bigger than a `DepNode<IndexIntoDefIdDirectory>` and our on-disk encoding of the dep-graph is inefficient, this PR will probably increase the amount of space the dep-graph takes up on disk. I'm in the process of gathering some performance data.
The changes in here are a step towards implementing ICH-based `DepNodes` (#42294).
r? @nikomatsakis