Support function specific dependencies

Assume we have 4 stages:

• Stages 0 and 2 depend on the function foo
• Stages 1 and 3 depend on the function bar

In order to reduce code duplication we want to put the utilities foo and bar in one place; let them be in utils.py. So, utils.py becomes a dependency of all four stages.

Now... after some time we realize we have to update something in bar. So we edit utils.py. This, in turn, invalidate all stages. Obviously, a simple solution is to have utils_0_2.py which will be a dependency of 0 and 2 and similarly utils_1_3.py. But this solution conflicts with good practices of coding.

A possible solution could be to adopt some notation like the one used in pytest and specify the function inside utils.py. For example, something like:

deps:
- md5: 409a438ee6ac70fc1f911631412325ea
  path: utils.py::foo

This is a continuation of this discussion.

Start by reviewing the linked discussion and the proposed deps notation for function-level paths such as utils.py::foo. Trace how dependencies invalidate stages, using the four-stage example as the expected behavior. Done means changes to foo invalidate only stages 0 and 2, while changes to bar invalidate only stages 1 and 3.