Use standard cache dir as default cacheDirectory

Set the default cache directory as the
[common cache directory](https://github.com/avajs/find-cache-dir),
`./node_modules/.cache/babel-loader`.

Previously, when `cacheDirectory` was set to `true`, babel-loader
tried to use the operating system's temporary directory as a cache
directory.

However, when running npm scripts as the root user,
[npm overrides the TMPDIR environment variable](npm/npm#4531).
This caused the cache files to be created in the project folder itself,
for example when using Docker: facebook/create-react-app#483.

lib/fs-cache.js

@@ -11,6 +11,7 @@
  */
 var crypto = require('crypto');
 var mkdirp = require('mkdirp');
+var findCacheDir = require('find-cache-dir');
 var fs = require('fs');
 var os = require('os');
 var path = require('path');
@@ -126,7 +127,7 @@ var cache = module.exports = function(params, callback) {
   var identifier = params.identifier;
   var directory = (typeof params.directory === 'string') ?
         params.directory :
-        os.tmpdir();
+        findCacheDir({ name: 'babel-loader' });
   var file = path.join(directory, filename(source, identifier, options));
 
   // Make sure the directory exists.

package.json

@@ -7,6 +7,7 @@
     "lib"
   ],
   "dependencies": {
+    "find-cache-dir": "^0.1.1",
     "loader-utils": "^0.2.11",
     "mkdirp": "^0.5.1",
     "object-assign": "^4.0.1"

test/cache.test.js

@@ -12,6 +12,8 @@ var webpack = require('webpack');
 describe('Filesystem Cache', function() {
   this.timeout(15000);
 
+  var defaultCacheDir = path.resolve(__dirname,
+    '../node_modules/.cache/babel-loader');
   var cacheDir = path.resolve(__dirname, 'output/cache/cachefiles');
   var outputDir = path.resolve(__dirname, './output/cache/');
   var babelLoader = path.resolve(__dirname, '../');
@@ -43,6 +45,9 @@ describe('Filesystem Cache', function() {
       mkdirp(cacheDir, done);
     });
   });
+  beforeEach(function(done) {
+    rimraf(defaultCacheDir, done);
+  });
 
   it('should output files to cache directory', function(done) {
 
@@ -73,7 +78,7 @@ describe('Filesystem Cache', function() {
     });
   });
 
-  it('should output files to OS\'s tmp dir', function(done) {
+  it('should output files to standard cache dir by default', function(done) {
     var config = assign({}, globalConfig, {
       module: {
         loaders: [
@@ -93,7 +98,7 @@ describe('Filesystem Cache', function() {
     webpack(config, function(err, stats) {
       expect(err).to.be(null);
 
-      fs.readdir(os.tmpdir(), function(err, files) {
+      fs.readdir(defaultCacheDir, function(err, files) {
         files = files.filter(function(file) {
           return /\b[0-9a-f]{5,40}\.json\.gzip\b/.test(file);
         });