CompiledPackageGroup as a repository

Signed-off-by: Jesse Zhang <jz@pivotallabs.com>
Remove comments

Signed-off-by: Jesse Zhang <jz@pivotallabs.com>
Force using `send_file` and the exporter in controller spec

Signed-off-by: Jesse Zhang <jz@pivotallabs.com>
CompiledPackagesExporter delegates to CompiledPackageDownloader and TarGzipper

Signed-off-by: Jesse Zhang <jz@pivotallabs.com>