Unbreak apps that reached a corrupted state due to #927

Previously to fixing #927, if you had a 'node_modules' directory
anywhere up the directory tree from your app and you had a package
in your app using NPM, all calls to 'npm' ran against the 'node_modules'
directory it found, so neither 'node_modules' nor 'npm-shrinkwrap.json'
would be created within your package's .npm directory.

The fix to #927 ensured that 'node_modules' was first created within
'.npm' but people who had already run 0.6.0 were still in a corrupted state.

tools/meteor_npm.js

@@ -62,6 +62,19 @@ _.extend(exports, {
     var newPackageNpmDir = packageNpmDir + '-new-' + self._randomToken();
 
     try {
+      // v0.6.0 had a bug that could cause .npm directories to be
+      // created without npm-shrinkwrap.json
+      // (https://github.com/meteor/meteor/pull/927). Running your app
+      // in that state causes consistent "Corrupted .npm directory"
+      // errors.
+      //
+      // If you've reached that state, delete the empty directory and
+      // proceed.
+      if (fs.existsSync(packageNpmDir) &&
+          !fs.existsSync(path.join(packageNpmDir, 'npm-shrinkwrap.json'))) {
+        files.rm_recursive(packageNpmDir);
+      }
+
       if (fs.existsSync(packageNpmDir)) {
         // we already nave a .npm directory. update it appropriately with some ceremony involving:
         // `npm install`, `npm install name@version`, `npm shrinkwrap`