Rollup merge of #67761 - cjgillot:split_graph, r=Zoxc

Move the dep_graph construction to a dedicated crate.

The interface for librustc consists in two traits: `DepKind` and `DepContext`.

The `DepKind` is the main interface. It allows to probe properties of the dependency.
As before, `DepNode` is the pair of a `DepKind` object and a hash fingerprint.

The `DepContext` takes the place of the `TyCtxt`, and handles communication with the query engine.

The use of the `ImplicitCtxt` through `ty::tls` is done through the `DepKind` trait.
This may not be the best choice, but it seemed like the simplest.