Make docs_from_files and docs_from_modules behave the same

`ExDoc.Retriever.docs_from_files` and `ExDoc.Retriever.docs_from_modules` handled modules with `@moduledoc false` differently. `docs_from_files` would return an empty list while `docs_from_modules` would return an `ExDoc.ModuleNode` with the `docs` field set to `[]`.

Both functions now return `[]`.

I ran into this difference while trying to remove the test fixtures from `ex_doc`. I can't think of a case where you want different behavior, but this might break code if someone was relying on it.