Use empty source map with worker-loader

Reverting a hack that was implemented in https://github.com/uber/fusionjs/pull/1489 to fix a module name collision in generated source maps, as modifying the import path by appending querystring is not compatible with the resolution logic specified for "exports" field, where the package consumers must use exact import paths according to how the package author has it defined in the "exports" field in the package.json. E.g., for [@duckdb/duckdb-wasm](https://github.com/duckdb/duckdb-wasm/blob/15c8075b5f82d3d32386c5c1cd425b5473524de6/packages/duckdb-wasm/package.json#L96-L146)
```
{
  "...": "",
  "exports": {
        "./dist/duckdb-browser-mvp.worker.js": "./dist/duckdb-browser-mvp.worker.js",
        "...": ""
  }
}
```

The import must take form of `import from '@duckdb/duckdb-wasm/dist/duckdb-browser-mvp.worker.js'`, and `import from '@duckdb/duckdb-wasm/dist/duckdb-browser-mvp.worker.js?workerUrl=true'` is not allowed

Here's Node.js's `require.resolve()` output:
```
Welcome to Node.js v16.15.0.

> require.resolve('@duckdb/duckdb-wasm/dist/duckdb-browser-mvp.worker.js')
'~/test-dd/node_modules/@duckdb/duckdb-wasm/dist/duckdb-browser-mvp.worker.js'

> require.resolve('@duckdb/duckdb-wasm/dist/duckdb-browser-mvp.worker.js?workerUrl=true')
Uncaught:
Error [ERR_PACKAGE_PATH_NOT_EXPORTED]: Package subpath './dist/duckdb-browser-mvp.worker.js?workerUrl=true' is not defined by "exports" in ~/test-dd/node_modules/@duckdb/duckdb-wasm/package.json
    at __node_internal_captureLargerStackTrace (node:internal/errors:465:5)
    at new NodeError (node:internal/errors:372:5)
    at throwExportsNotFound (node:internal/modules/esm/resolve:472:9)
    at packageExportsResolve (node:internal/modules/esm/resolve:753:3)
    at resolveExports (node:internal/modules/cjs/loader:482:36)
    at Function.Module._findPath (node:internal/modules/cjs/loader:522:31)
    at Function.Module._resolveFilename (node:internal/modules/cjs/loader:919:27)
    at Function.resolve (node:internal/modules/cjs/helpers:108:19) {
  code: 'ERR_PACKAGE_PATH_NOT_EXPORTED'
}
```

The original issue seems to be fixable by resolving the `worker-loader` using empty source map for the "virtual" module it generates.

fusion-cli-tests/test/e2e/worker/test.js

@@ -2,6 +2,7 @@
 /* eslint-env node */
 
 const t = require('assert');
+const fs = require('fs');
 const path = require('path');
 
 const puppeteer = require('puppeteer');
@@ -46,3 +47,42 @@ test('`fusion build` app with worker integration', async () => {
   await browser.close();
   proc.kill('SIGKILL');
 }, 100000);
+
+test('`fusion build` worker loader client source maps', async () => {
+  const dir = path.resolve(__dirname + '/fixture');
+
+  const env = Object.create(process.env);
+  env.NODE_ENV = 'production';
+
+  await cmd(`build --dir=${dir} --production --modernBuildOnly`, {env});
+
+  const clientDistPath = path.join(
+    dir,
+    '.fusion',
+    'dist',
+    'production',
+    'client'
+  );
+
+  const clientSourceMapFiles = (
+    await fs.promises.readdir(clientDistPath)
+  ).filter((filePath) => filePath.endsWith('.map'));
+
+  const sourceMaps = (
+    await Promise.all(
+      clientSourceMapFiles.map((filePath) =>
+        fs.promises.readFile(path.join(clientDistPath, filePath), 'utf-8')
+      )
+    )
+  ).map((fileContents) => JSON.parse(fileContents));
+
+  const sources = sourceMaps.reduce((acc, sourceMap) => {
+    return [...acc, ...sourceMap.sources];
+  }, []);
+  const uniqueSources = new Set(sources);
+
+  t(
+    uniqueSources.size === sources.length,
+    'Generated source maps should not contain duplicate entries for the same source file'
+  );
+}, 100000);

fusion-cli/build/babel-plugins/babel-plugin-workerurl/index.js

@@ -42,9 +42,7 @@ function refsHandler(t, context, refs = []) {
     }
     args[0].replaceWith(
       t.callExpression(t.identifier('require'), [
-        t.stringLiteral(
-          `__SECRET_WORKER_LOADER__!${args[0].node.value}?workerUrl=true`
-        ),
+        t.stringLiteral(`__SECRET_WORKER_LOADER__!${args[0].node.value}`),
       ])
     );
   });

fusion-cli/build/babel-plugins/babel-plugin-workerurl/test/fixtures/expected-import-destructuring

@@ -1,6 +1,6 @@
 import { workerUrl } from 'fusion-core';
 import { workerUrl as workerUrlOther } from 'workerUrl';
 const path = './test';
-workerUrl(require("__SECRET_WORKER_LOADER__!./path?workerUrl=true"));
+workerUrl(require("__SECRET_WORKER_LOADER__!./path"));
 workerUrlOther(path);
-workerUrl(require("__SECRET_WORKER_LOADER__!./path?workerUrl=true"));
+workerUrl(require("__SECRET_WORKER_LOADER__!./path"));

fusion-cli/build/babel-plugins/babel-plugin-workerurl/test/fixtures/expected-import-destructuring-as

@@ -1,4 +1,4 @@
 import { workerUrl as frameworkWorkerUrl } from 'fusion-core';
 import workerUrl from 'workerURL';
 workerUrl('./path');
-frameworkWorkerUrl(require("__SECRET_WORKER_LOADER__!./path?workerUrl=true"));
+frameworkWorkerUrl(require("__SECRET_WORKER_LOADER__!./path"));

fusion-cli/build/loaders/worker-loader.js

@@ -24,6 +24,15 @@ class WorkerLoaderError extends Error {
   }
 }
 
+const EMPTY_SOURCE_MAP = {
+  file: 'x',
+  version: 3,
+  names: [],
+  sources: [],
+  sourcesContent: [],
+  mappings: '',
+};
+
 const getWorker = (file, content, options) => {
   const publicPath = options.publicPath
     ? JSON.stringify(options.publicPath)
@@ -122,7 +131,14 @@ module.exports.pitch = function (request /* : any*/) {
         delete this._compilation.assets[worker.file];
       }
 
-      return cb(null, `module.exports = ${worker.factory};\n`);
+      return cb(
+        null,
+        `module.exports = ${worker.factory};\n`,
+        // Returning an empty source map for the "virtual" module created here,
+        // fixes a naming collision with the actual source map generated for the
+        // bundled web worker script
+        EMPTY_SOURCE_MAP
+      );
     }
 
     return cb(null, null);