extends: build a vinca.yaml on top of a shared one

[Tobias-Fischer]

This lets several configurations share one base, e.g. one vinca.yaml per ROS 2 distribution on top of a shared one, as RoboStack's monorepo does:

# distros/jazzy/vinca.yaml
extends: ../../shared/vinca.yaml
ros_distro: jazzy
mutex_package:
  version: 0.19.0
  run_constraints:
    - libprotobuf 7.35.*   # replaces the shared libprotobuf constraint

How the layers combine:

• packages_select_by_deps, packages_exclude and packages_skip are combined, base first.
• mutex_package is merged: the extending file's keys win, and its run_constraints replace the base's constraint on the same package and add the others.
• Any other key of the extending file replaces the base's.
• The files next to each configuration are combined:
  • pkg_additional_info.yaml and <patch_dir>/dependencies.yaml: per package, the extending file's keys win;
  • patches and tests/: per package, the extending file's file wins;
  • conda_index files: the extending file's mappings are looked up first.
• A base can itself extend another file, and cycles are rejected.

Paths (patch_dir, the snapshot paths, conda_index files) are now resolved relative to the file that sets them, rather than the working directory. vinca is normally run in the directory of its vinca.yaml, so that resolves to the same paths. vinca-gha's additional-recipes check now reads the layered configuration, so skip_existing from a base is seen too.

Tests: six new tests cover lists, scalars, selectors, the mutex merge, per-layer files, file-relative paths and cycle detection. All 267 tests pass, ruff and pyrefly are clean, and the README documents extends.

Checked against RoboStack: RoboStack's monorepo previously did this merging in its own scripts. With this branch, its distributions' vinca.yaml files just extend the shared one, and the generated recipes are identical (rolling, lyrical, jazzy and humble on linux-64, and humble on win-64).

🤖 Generated with Claude Code

[traversaro]

Builds on #162, so its first two commits are #162's. Please review and merge #162 first.

That PR has been merged, can you update the PR description?

[Tobias-Fischer]

Builds on #162, so its first two commits are #162's. Please review and merge #162 first.

That PR has been merged, can you update the PR description?

Done :). Would you mind taking a quick look please @traversaro?