don't use global definition in standalone gen

bundler was using the global 'Bundler.definition' for
standalone, but the local definition elsewhere. This was
getting in the way of using bundler from within an application
to generate a standalone bundle if bundler was being used
to bootstrap the application in the first place.
Merge pull request #2651 from cowboyd/use-instance-definition-in-installer

don't use global definition in standalone gen