Simplify function resolution/check and git logging

[eywalker]

• The shared logic for performing function resolution is not placed inside builder and any function resolution is performed against the namespace of the builder module, matching the fact that when actually loading, that's where the name resolution is performed. Consequently, .models, .datasets and .training are no longer loaded inside main.py
• Functionality of performing Git logging is not distilled into a separate decorator, so that the handling of git commits etc are not tightly coupled into the implementation of make method as was found in TrainedModel. This also means anyone can easily make use of the new gitlog decorator to perform logging of git repos.

These are subtle but quite substantial changes and would really like thorough review by the requested individuals.