`lake graph`

As discussed previously in the Zulip it would be beneficial if Lake could output a graph representation of the dependencies it has discovered. In order to replace the Lake dependency in doc-gen4 it would need to contain:

• The git hash of the Lean compiler used for this library which is IMO technically a dependency (doc-gen checks if this is equivalent to its own and outputs a warning if compiler versions don't match up since this could cause ABI issues etc.)
• The git remote URLs of all dependencies
• A map from dependency name to the modules they contain

Furthermore it also requires:

• The contents of ws.root.srcDir and ws.leanSrcPath which I use to search the source files LeanInk should highlight.
• A way to obtain Lake's search path

but afaik those are already covered by lake print-paths

Start by examining the existing lake print-paths output and how lake graph is expected to expose dependency information for doc-gen4. Define what the graph must report: the Lean compiler git hash, dependency remotes, each dependency's modules, and the existing source and search paths; done means all listed data is obtainable through Lake.