Defining package imports via app.json

[mellistibco]

Current behavior: Currently, there is no easy way to define the package imports required for contributions other than activities and triggers. Today, mapper functions, as an example, are all available within the flogo core package. This is clearly not ideal.

Expected behavior: As we look to expose expression/mapper function contributions, we need a simplified mechanism to allow developers to import the required packages. One consideration is to expose some sort of import property in the app.json, this would enable app developers to use expression/mapper functions from any repo and the flogo CLI will just bring in the import statements when the imports.go is generated.

What is the motivation / use case for changing the behavior? Enable app devs to easily define additional expression/mapper function contributions. Also related to #215

Additional information you deem important (e.g. I need this tomorrow):

[mellistibco]

Another point, if extended past just expression/mapper functions and more generally just imports, the inference activity can support additional frameworks in a simplified fashion.