Multi-repository dependency management

[alessandrothea]

The lack of a mechanism to semi-automatically setup a multi-repo work area is probably one of the longest-standing issues in ipbb. The problem is twofold:

• How to define dependencies.
  Using .ipbb_setup.yml is probably the simplest solution.
• Properly handling multi-layer dependencies.

Implementing a proper dependency management schema seems overkill, for a few reasons.
It could be a lot of work and concepts like 'minimal required package version' require some basic tag conventions, which are not guaranteed.

Instead, the most reasonable way forward is a 'limited responsibility' revision management schema. In this scenario, only a minimal set of automatic checks and actions are implemented to handle multi-layer dependencies and the user intervention is requested in case of any minimal conflict or inconsistency.

For example, let's assume to have 3 inter-dependant repositories A, B and C (and D for the last case)

1. simple case: A depends on B that depends on C. A imports B, but puts no requirements on C. C's revision is taken from B. (dependency imports)
2. simple conflict: A depends on B and a specific C version. A conflict between A's and B's reqs on C arises. Assuming that the top-level is in A, a warning is thrown, but the revision of C specified by A is taken (top package dependencies prevail)
3. sup-package conflict: A depends on B and C, both conflictual depending on D. The responsibility to choose the correct version of C is delegated to A (top package resolves conflicts)

This approach is rather straightforward to implement and avoid inconsistent situations.