evolve: move builddependencies() and _singlesuccessor() to utility module The builddependencies() and _singlesuccessor() function are very useful functions and can be used at a lot of places. They are also used in topic extension also, so let's move them to utility. However moving them to utility won't help anything in the topic extension.

utility: move MultipleSuccessorsError from __init__.py The MultipleSuccessorsError is required in multiple modules in upcoming patches, so let's move it to utility first.

evolvecmd: move more core from __init__.py to evolvecmd.py We are done with moving helper code, we will now be moving the evolve command in the new module.