Allow chainingh package loaders

Currently we have Combined Api and Directory package loaders.

Combined has several package loaders, that allow setting several directories to look for packages and exactly one API loader. Directories always have higher priority.

We can modify this situation to have chanining package-loaders. i.e. check 1st package-loader, then 2d, etc. in the order they're mentioned in config. This would allow us to have 1 engine listen to several API's and read from several directories and set arbitrary priority for those loaders