Feature: View module dependency graph

It would be nice if we could show a graph for a given crate that visualizes the interdepndencies between the crate's modules. That is, if module a imports/references something from module b we would draw an edge between the two showing this dependency.

In Rust it is rather easy to accidentally create a very messy setup where modules in a crate suddenly refer in plenty of cyclic ways making it very difficult to follow. The feature here would help showing the problem and possibly aid in figuring out how/what to restructure to reduce the cyclic dependencies between them. What's nice about removing these kinds of module dependencies in a crate is that it allows extracting subcrates from them as a module that is only referred to by other modules can be simply extracted from a crate without problems (in most cases).

The issue names no files, tests, or entry points, so first determine where rust-analyzer could collect module imports and how a crate graph should be presented. Define the graph scope and output format before implementation; done means a given crate's module dependencies are visualized with edges, including cyclic relationships.