chore(import-graph): don't overwrite 'PR' colour with 'ready' colour

[semorrison]

The colours scheme in import-graph makes more sense if the "PR" status takes precedence of the "ready" colour.

[semorrison]

Oh, I hadn't seen #132. This PR is subsumed by that one, although I don't immediately understand what else it is doing!

[joneugster]

Please go ahead and merge this PR before #132! I didn't realise it would include any commits I made to my mathlib-tool fork automatically into the PR and used the same fork to do something different.

I might close #132, I use --include-deps locally on a non-mathlib project to show me which imports of mathlib are needed for it to be Lean4-ready. It goes recursively into any dependencies outside the project and includes them into the import-graph, too.