fix(cjs-build): enable evaluating import.meta in cjs build (#1236)

## Purpose

A while ago, a [PR](#897) was merged
to disable `import.meta` evaluation altogether to support new versions
of Webpack. This sometimes causes issues in CJS projects.

In the comments, there were some
[discussions](#897 (comment))
about whether we should have a conditional and keep the previous
behavior for CJS builds. This PR addresses exactly this by translating
`import.meta.url` to a CJS compatible statement which results to the
same output.

I have created a [comparative
build](https://github.com/meienberger/ncc/pull/1/files) between two
projects using the same code and a dependency (@sentry/node) that uses
`import.meta.url`. You can see the difference in the output `index.js`

Before the change:

```javascript
const importMetaUrl = typeof import.meta.url !== 'undefined' ? import.meta.url : undefined;
```

After:

```javascript
const importMetaUrl = typeof require("url").pathToFileURL(__filename).href !== 'undefined' ? require("url").pathToFileURL(__filename).href : undefined;
```

Closes #1019

Author: nicotsx

src/index.js

@@ -252,6 +252,12 @@ function ncc (
     }));
   }
 
+  if (!esm) {
+    plugins.push(new webpack.DefinePlugin({
+      'import.meta.url': 'require("url").pathToFileURL(__filename).href'
+    }));
+  }
+
   const compiler = webpack({
     entry,
     cache: cache === false ? undefined : {

test/unit/import-meta-cjs/input.js

@@ -0,0 +1 @@
+console.log(import.meta.url);

test/unit/import-meta-cjs/output-coverage.js

@@ -0,0 +1,10 @@
+/******/ (() => { // webpackBootstrap
+/******/ 	/* webpack/runtime/compat */
+/******/ 	
+/******/ 	if (typeof __nccwpck_require__ !== 'undefined') __nccwpck_require__.ab = __dirname + "/";
+/******/ 	
+/************************************************************************/
+var __webpack_exports__ = {};
+console.log(require("url").pathToFileURL(__filename).href);
+/******/ })()
+;

test/unit/import-meta-cjs/output.js

@@ -0,0 +1,11 @@
+/******/ (() => { // webpackBootstrap
+/******/ 	/* webpack/runtime/compat */
+/******/ 	
+/******/ 	if (typeof __nccwpck_require__ !== 'undefined') __nccwpck_require__.ab = __dirname + "/";
+/******/ 	
+/************************************************************************/
+var __webpack_exports__ = {};
+console.log(require("url").pathToFileURL(__filename).href);
+module.exports = __webpack_exports__;
+/******/ })()
+;