addInternalAddon(internalAddonPkg) {
    if (!this.internalAddons) {
      this.internalAddons = [];
    }
    this.internalAddons.push(internalAddonPkg);
  }
Add a reference to an internal addon PackageInfo object.
"internal" addons (note: not in-repo addons) only exist in
Projects, not other packages. Since the cache is loaded from
'loadProject', this can be done appropriately.

@protected
@method addInternalAddon
@param {PackageInfo} internalAddonPkg the PackageInfo for the internal addon
@return null