modules: busts up webpack config

Existence of a nested `package.json` prevents an ancestor's webpack config from being found.

The issue names no files, tests, or entry points. Start by reproducing the behavior with a nested package.json and an ancestor webpack config, then trace how the configuration is located. Done means the ancestor webpack config is found despite the nested package.json, with a regression test covering the case.

Written by the indexing model from the issue text.