discoverAddons() {
    if (this._didDiscoverAddons) {
      return;
    }
    this._didDiscoverAddons = true;

    let addonPackageList = this._packageInfo.discoverProjectAddons();
    this.addonPackages = this._packageInfo.generateAddonPackages(addonPackageList);

    // in case any child addons are invalid, dump to the console about them.
    this._packageInfo.dumpInvalidAddonPackages(addonPackageList);
  }
Discovers all addons for this project and stores their names and
package.json contents in this.addonPackages as key-value pairs.

Any packageInfos that we find that are marked as not valid are excluded.

@private
@method discoverAddons