Use public api, move functions into public api

@forki I worked a bit on https://github.com/fsprojects/Paket/issues/341 - please offer guidance and I can continue if you like the direction. 

Thanks, 

Brody 

Need review on:

* Should these be public APIs?
* What should be done with bool flags?
This line would break former functionality when no command was given, as it now always try to locate the dependencies file and can fail if it's not present. Please make it lazy again.