Add assets source filenames

Asset resources in `webpack-stats.json` file will look like,

```json
"assets": {
  "assets/test-bbf3c94e2a3948c98900.txt": {
    "name": "assets/test-bbf3c94e2a3948c98900.txt",
    "path": "/home/user/project-root/assets/test-bbf3c94e2a3948c98900.txt",
    "publicPath": "http://localhost:3000/assets/test-bbf3c94e2a3948c98900.txt",
    "sourceFilename": "src/test.txt"
  }
}
```

Related to django-webpack/django-webpack-loader#343

lib/index.js

@@ -192,6 +192,8 @@ class BundleTrackerPlugin {
         fileInfo.path = path.relative(this.outputChunkDir, fileInfo.path);
       }
 
+      fileInfo.sourceFilename = stats.compilation.assetsInfo.get(assetName).sourceFilename;
+
       output.assets[assetName] = fileInfo;
     });
     each(stats.compilation.chunkGroups, chunkGroup => {

tests/fixtures/appWithAssetResources.js

@@ -0,0 +1,9 @@
+"use strict";
+
+const common = require('./commons');
+
+require('./assets/resources/test.txt')
+
+require(["./shared"], function(shared) {
+	shared("This is app with asset resources");
+});

tests/fixtures/assets/resources/test.txt

@@ -0,0 +1 @@
+Hello world

tests/webpack5.test.js

@@ -638,6 +638,53 @@ describe('BundleTrackerPlugin bases tests', () => {
     );
   });
 
+  it("shows original asset's filepath", done => {
+    const expectErrors = null;
+    const expectWarnings = getWebpack5WarningMessage();
+
+    testPlugin(
+      webpack5,
+      {
+        context: __dirname,
+        entry: {
+          appWithAssetResources: path.resolve(__dirname, 'fixtures', 'appWithAssetResources.js'),
+        },
+        output: {
+          assetModuleFilename: 'assets/[name]-[contenthash][ext]',
+          path: OUTPUT_DIR,
+          filename: 'js/[name].js',
+          publicPath: 'http://localhost:3000/assets/',
+        },
+        module: {
+          rules: [{ test: /\.txt$/, type: 'asset/resource' }],
+        },
+        plugins: [
+          new BundleTrackerPlugin({
+            path: OUTPUT_DIR,
+            relativePath: true,
+            includeParents: true,
+            filename: 'webpack-stats.json',
+          }),
+        ],
+      },
+      {
+        status: 'done',
+        assets: {
+          'assets/test-bbf3c94e2a3948c98900.txt': {
+            name: 'assets/test-bbf3c94e2a3948c98900.txt',
+            path: 'assets/test-bbf3c94e2a3948c98900.txt',
+            publicPath: 'http://localhost:3000/assets/assets/test-bbf3c94e2a3948c98900.txt',
+            sourceFilename: 'fixtures/assets/resources/test.txt',
+          },
+        },
+      },
+      'webpack-stats.json',
+      done,
+      expectErrors,
+      expectWarnings,
+    );
+  });
+
   it('correctly merges chunks after multiple runs', done => {
     fs.writeFileSync(
       path.join(OUTPUT_DIR, 'app1.js'),