Validation should follow descendents

When validating a graph and reaching a type branch, current behaviour descends into the children imports, treating them as code imports, and therefore erroring when there is a type branch only error, which it shouldn't.

See: https://github.com/denoland/deno/issues/15295

Start by reading the linked Deno issue 15295 and tracing validation when the graph reaches a type branch. The payload names no source files, tests, or entry points, so first identify the validation path and its existing reproduction; done means type-only descendants no longer produce code-import validation errors.

Written by the indexing model from the issue text.