Crashes webpack when used in conjunction with pnpIgnorePatterns

[andreialecu]

Repro at: https://github.com/andreialecu/repro-yarn-nm-pnp

nm-packages is under pnpIgnorePatterns. Trying to run yarn start:nm fails with:

[error] Error: the specified module doesn't seem to be part of the dependency tree
    at getModuleLocator (/home/aandrei/projects/repros/yarn-nm-pnp/nm-packages/angular/node_modules/pnp-webpack-plugin/index.js:21:11)
    at Object.module.exports.moduleLoader (/home/aandrei/projects/repros/yarn-nm-pnp/nm-packages/angular/node_modules/pnp-webpack-plugin/index.js:140:23)
    at Object.getCommonConfig (/home/aandrei/projects/repros/yarn-nm-pnp/nm-packages/angular/node_modules/@angular-devkit/build-angular/src/angular-cli-files/models/webpack-configs/common.js:390:40)
    at webpackPartialGenerator (/home/aandrei/projects/repros/yarn-nm-pnp/nm-packages/angular/node_modules/@angular-devkit/build-angular/src/browser/index.js:59:27)
    at generateWebpackConfig (/home/aandrei/projects/repros/yarn-nm-pnp/nm-packages/angular/node_modules/@angular-devkit/build-angular/src/utils/webpack-browser-config.js:63:22)
    at async generateBrowserWebpackConfigFromContext (/home/aandrei/projects/repros/yarn-nm-pnp/nm-packages/angular/node_modules/@angular-devkit/build-angular/src/utils/webpack-browser-config.js:154:20)
    at async Object.generateI18nBrowserWebpackConfigFromContext (/home/aandrei/projects/repros/yarn-nm-pnp/nm-packages/angular/node_modules/@angular-devkit/build-angular/src/utils/webpack-browser-config.js:95:20)
    at async setup (/home/aandrei/projects/repros/yarn-nm-pnp/nm-packages/angular/node_modules/@angular-devkit/build-angular/src/dev-server/index.js:78:47)

pnp-webpack-plugin/index.js

Lines 15 to 16 in 659125c

	if (!moduleLocation)
	throw new Error(`The specified module doesn't seem to exist on the filesystem`);

will be null if the path is under a pnpIgnorePattern path as per yarnpkg/berry#2055

I guess instead of a hard crash there should be a fallback to plain webpack resolution.