Better detection of node modules when pruning

Fixes #845.

prune.js

@@ -59,6 +59,7 @@ class Pruner {
 module.exports = {
   isModule: function isModule (pathToCheck) {
     return fs.pathExists(path.join(pathToCheck, 'package.json'))
+      .then(exists => exists && path.basename(path.dirname(pathToCheck)) === 'node_modules')
   },
   Pruner: Pruner
 }

test/prune.js

@@ -2,6 +2,8 @@
 
 const fs = require('fs-extra')
 const path = require('path')
+const prune = require('../prune')
+const test = require('ava')
 const util = require('./_util')
 
 function checkDependency (t, resourcesPath, moduleName, moduleExists) {
@@ -53,3 +55,11 @@ util.testSinglePlatform('prune electron in dependencies', (t, baseOpts) => {
 
 util.testSinglePlatform('prune: false test', createPruneOptionTest, false,
                         'package.json devDependency should exist under app/node_modules')
+
+test('isModule only detects modules inside a node_modules parent folder', t =>
+  prune.isModule(util.fixtureSubdir(path.join('prune-is-module', 'node_modules', 'module')))
+    .then(isModule => {
+      t.true(isModule, 'module folder should be detected as module')
+      return prune.isModule(util.fixtureSubdir(path.join('prune-is-module', 'node_modules', 'module', 'not-module')))
+    }).then(isModule => t.false(isModule, 'not-module folder should not be detected as module'))
+)