Add nodes with no edges to the dependency graph

[janmasrovira]

• Closes Type inference crashes in lets for numeric types #2373

Consider this:

let 
 x : _ := 0
in ...

When translating the let to internal, we build the dependency graph and then use that to group definitions in mutually recursive blocks. Since x has no edge, it was not being added to the dependency graph, so it was not being translated to Internal, thus crashing later during inference.