Managing cross cutting dependencies in lively core packages

**Describe the Feature**
Currently there are a lot of duplicate dependencies found within each of the lively.next core packages.
For instance SystemJS can be found referenced under the very same version in `lively.modules`, 'lively.freezer' and others.

**Implementation Details**
Based on our recent discussions there are 2 ways out of this:
1.) The "proper" way would be, to place an enclosing package.json into the lively.next project, thereby declaring lively.next itself to the "super" package that comprises all of the core package. It is not clear wether or not flatn is able to cope with that, which may complicate things.
2.) A simple config file, that for the time being would serve as an ad hoc solution to this problem until we investigate the above way in more detail.

Start by comparing how SystemJS is declared in the lively.modules, lively.freezer, and other core packages, then investigate whether flatn supports an enclosing package.json for lively.next. Define the chosen package-level or config-file approach and verify that it removes duplicate dependency declarations without breaking package management.

Written by the indexing model from the issue text.