Fix: Correctly load v8-compile-cache (#6261)

`v8-compile-cache` is not loading because `fs.existsSync` is checking for a file that doesn't exist. `fs.existsSync` resolves relative to the cwd, not the current module (like `require`), and `v8CompileCachePath`'s filename is missing `.js`.

Instead of checking for the existence of `v8-compile-cache.js`, just try to `require` it in a `try`/`catch`, using a full path. Also switched the `require` of `cli.js` to use the same pattern for consistency (plus it's a module resolution micro-optimization).

```
# before
$ time ./dist/bin/yarn -v
1.10.0-0
./dist/bin/yarn -v  0.28s user 0.06s system 98% cpu 0.344 total

# after
$ time ./dist/bin/yarn -v
1.10.0-0
./dist/bin/yarn -v  0.16s user 0.05s system 97% cpu 0.213 total
```

bin/yarn.js

@@ -11,18 +11,17 @@ if (majorVer < 4) {
  console.error('Node version ' + ver + ' is not supported, please use Node.js 4.0 or higher.');
  process.exit(1); // eslint-disable-line no-process-exit
 } else {
- var dirPath = '../lib/';
- var v8CompileCachePath = dirPath + 'v8-compile-cache';
- var fs = require('fs');
- // We don't have/need this on legacy builds and dev builds
- if (fs.existsSync(v8CompileCachePath)) {
- require(v8CompileCachePath);
+ try {
+ require(__dirname + '/../lib/v8-compile-cache.js');
+ } catch (err) {
+ // We don't have/need this on legacy builds and dev builds
  }
 
  // Just requiring this package will trigger a yarn run since the
  // `require.main === module` check inside `cli/index.js` will always
  // be truthy when built with webpack :(
- var cli = require(dirPath + 'cli');
+ // `lib/cli` may be `lib/cli/index.js` or `lib/cli.js` depending on the build.
+ var cli = require(__dirname + '/../lib/cli');
  if (!cli.autoRun) {
  cli.default().catch(function(error) {
  console.error(error.stack || error.message || error);