Feature: smart order of cargo diagnostics

It seems that currently diagnostics are sorted by file path. This order isn't very useful when one wants to refactor some low-level modules (and their API is changed), because higher-level modules get errors too, and these errors are unactionable (One can't fix high-level code because they haven't updated low-level code yet).

I'd propose the following algorithm to determine better readable diagnostic order:

1. Build an oriented graph, where each vertex corresponds to a source file, and edge u -> v exists if the file u references file v (e.g. mentions an item defined in v).
2. Find any topological sorting of this graph.
3. When return diagnostics, sort them using source file's position in the sorting as a comparison key.

Start by tracing where rust-analyzer currently orders diagnostics by source-file path. Study how file-reference relationships could provide the directed graph, then determine how to handle cycles and disconnected files. Done means diagnostics are returned in a useful topological order so lower-level errors precede dependent higher-level errors.