Ability to walk the graph and tell when a module resolution error occurs due to a dynamic import

We need a way to use `.walk` and tell if an error is for a dynamic import that's also not imported via a regular import.

Also, this seems slightly wrong:

https://github.com/denoland/deno_graph/blob/9dbc98a1393a22fd10138f82b64eb7c1de0da1a8/src/graph.rs#L1090C33-L1094

Start with the `.walk` API and the referenced logic in `src/graph.rs` around lines 1090-1094. Determine how module-resolution errors are represented and how to distinguish a dynamic import that is not also reached through a regular import. Done means callers walking the graph can reliably identify that case and the referenced behavior is corrected.

Written by the indexing model from the issue text.