Support for PACKAGE_DIRS environment variable

[strikeout]

npm-container always set in ./packages, ignoring process.env.PACKAGE_DIRS

meteor-core uses this approach:

  // User can provide additional package directories to search in
  // PACKAGE_DIRS (colon-separated).
  _.each(process.env.PACKAGE_DIRS.split(':'), function (p) {
    searchDirs.push(files.pathResolve(p));
  });