Feature request: Expose source code location for Failure.Extern missing modules

[jonahbeckford]

In the following snippet:

let (map: ('a Solver.i * Solver.Failure.status option ref) Nms.map), _cmap =
    Solver.Failure.analyze block resolver pending

when the Solver.Failure.status matches | Solver.Failure.Extern (path: Namespaced.t)

there does not seem to be enough information to report the location (line number, column) of the missing Extern path.
(More accurately, the line/col for one of the missing module references).

At best the pseudocodelet {Solver.input; _} = (i: 'a Solver.i) can be used to find the filename with Pkg.filename input.src.