fix: error on specifiers that fail to resolve

[dsherret]

deno_graph's module_errors() explicitly does not include resolution errors, so a specifier that failed to resolve was silently emitted into the output as-is, which is broken for anything that isn't an npm dependency:

// input
import * as path from "@std/path";
import { globToRegExp } from "@std/path/glob-to-regexp";

// output
import * as path from "./deps/jsr.io/@std/path/1.1.6/mod.js";
import { globToRegExp } from "@std/path/glob-to-regexp"; // <-- broken

Now the graph is also walked for resolution errors, which surfaces the same message Deno gives:

error: Import "@std/path/glob-to-regexp" not a dependency and not in import map from "file:///.../util.ts"
    at file:///.../util.ts:2:30

The walk uses GraphKind::All and follow_dynamic: true rather than the narrower semantics of graph.valid(), because dnt emits type-only and dynamic imports too, so an unresolved specifier in either position is broken in the output in exactly the same way.

In the case from the issue the specifier failed to resolve because the deno.json was passed as importMap rather than configFile. Like deno --import-map, that is read as a plain import map, so bare specifier mappings are not expanded to cover sub paths (deno run --import-map=deno.json fails on the same import). Documented that on the option.

Closes #475