Incorporate packages to target platform definition

Currently a target platform in Studio defines:

• available features (and related builders), e.g items in the view
• builtin contributions (document types, action categories, etc...)

It should also list available packages.

Packages need to be defined independently and will be similar to target platforms (define contributions, builders, etc...)

The contributions should be merged when retrieving a target platform contribution.

Note that some contributions need advanced merging features that are not present on Studio side right now.
For example, DAM relies on a facet to add its features to existing document types: when extending the "File" document type for DAM, it will have to hold the "Asset" facet => find a way to merge contributions like Nuxeo merges its types in case another addon also adds facets to default types