add package_postinstall_bundle convenience attribute

The new packages promises in #6404 require post-install configuration to be done as a second bundle call. Instead, I propose to introduce a special @package_postinstall_bundle@ similar to the @users@ promises' @home_bundle@ and with the same call semantics (call on success; get promise details from the @this@ context).

If this is acceptable, I can try to write a patch...