Fix: avoid using process.exit() when possible (#3955)

**Summary**

Refs #3524. We are using `process.exit()` pretty liberally although it is [strongly advised not to use it](https://nodejs.org/api/process.html#process_process_exit_code) since it may cause the `stdout` to get corrupted/terminated before a full flush. This patch changes all possible `process.exit(code)` calls with `process.exitCode = code` statements.

We'd also ideally enable [no-process-exit](http://eslint.org/docs/rules/no-process-exit) rule in ESLint but it requires an upgrade to ESLint v4 which should be handled separately.

**Test plan**

Expect tests to pass and actually finish (not run indefinitely due to `yarn` not exiting at all). Also, the script referenced in #3524 to output something like the following:
```
DATA 18
DATA 8192
DATA 8192
DATA 5392
DATA 15
EXIT 0
```

Author: BYK

bin/yarn-bundle-entry.js

@@ -5,3 +5,12 @@
 
 require('../lib/v8-compile-cache');
 module.exports = require('../lib/yarn-cli');
+const main = module.exports.default;
+
+// ignore all arguments after a --
+const doubleDashIndex = process.argv.findIndex(element => element === '--');
+const startArgs = process.argv.slice(0, 2);
+const args = process.argv.slice(2, doubleDashIndex === -1 ? process.argv.length : doubleDashIndex);
+const endArgs = doubleDashIndex === -1 ? [] : process.argv.slice(doubleDashIndex + 1, process.argv.length);
+
+main({startArgs, args, endArgs});

bin/yarn.js

@@ -31,3 +31,12 @@ var constants = require(dirPath + '/constants');
 mkdirp.sync(constants.MODULE_CACHE_DIRECTORY);
 
 module.exports = require(dirPath + '/cli');
+const main = module.exports.default;
+
+// ignore all arguments after a --
+const doubleDashIndex = process.argv.findIndex(element => element === '--');
+const startArgs = process.argv.slice(0, 2);
+const args = process.argv.slice(2, doubleDashIndex === -1 ? process.argv.length : doubleDashIndex);
+const endArgs = doubleDashIndex === -1 ? [] : process.argv.slice(doubleDashIndex + 1, process.argv.length);
+
+main({startArgs, args, endArgs});

scripts/build-webpack.js

@@ -35,6 +35,8 @@ const compiler = webpack({
   output: {
     filename: `yarn-${version}.js`,
     path: path.join(basedir, 'artifacts'),
+    library: 'yarn-cli',
+    libraryTarget: 'commonjs2',
   },
   target: 'node',
 });
@@ -71,6 +73,8 @@ const compilerLegacy = webpack({
   output: {
     filename: `yarn-legacy-${version}.js`,
     path: path.join(basedir, 'artifacts'),
+    library: 'yarn-cli',
+    libraryTarget: 'commonjs2',
   },
   target: 'node',
 });