Fix mini-css-extract-plugin 0.4.3 issue

mini-css-extract-plugin reports additional, incorrect information for files that are refenced in CSS. The first time we see the file the `module.userRequest` is correct, and we add to `moduleAssets` correctly. However mini-css-extract-plugin then also reports the same `file` but with `module.userRequest` set to the CSS file that references it, which caused us to overwrite the good value in `moduleAssets`.

See the change in mini-css-extract-plugin that caused this webpack-contrib/mini-css-extract-plugin#177

fixes #167

lib/plugin.js

@@ -49,6 +49,22 @@ ManifestPlugin.prototype.apply = function(compiler) {
     }
   };
 
+  // TODO: check with @evilebottnawi if this is still needed for webpack@5
+  var normalModuleLoader = function (loaderContext, module) {
+    const { emitFile } = loaderContext;
+
+    loaderContext.emitFile = (file, content, sourceMap) => {
+      if (module.userRequest && !moduleAssets[file]) {
+        moduleAssets[file] = path.join(
+          path.dirname(file),
+          path.basename(module.userRequest)
+        );
+      }
+
+      return emitFile.call(module, file, content, sourceMap);
+    };
+  };
+
   var emit = function(compilation, compileCallback) {
     const emitCount = emitCountMap.get(outputFile) - 1
     emitCountMap.set(outputFile, emitCount);
@@ -214,7 +230,7 @@ ManifestPlugin.prototype.apply = function(compiler) {
   compiler.hooks.webpackManifestPluginAfterEmit = new SyncWaterfallHook(['manifest']);
 
   compiler.hooks.compilation.tap(pluginOptions, function (compilation) {
-    compilation.hooks.moduleAsset.tap(pluginOptions, moduleAsset);
+    compilation.hooks.normalModuleLoader.tap(pluginOptions, normalModuleLoader);
   });
   compiler.hooks.emit.tap(pluginOptions, emit);