Fall back to the rosdistro cache for snapshots without dependency metadata

[Tobias-Fischer]

Since snapshots started recording dependencies, vinca raises RuntimeError: Snapshot metadata for '<pkg>' has no dependencies; regenerate the rosdistro snapshot for any snapshot written before then. That is the case for ros-rolling and ros-humble today. They can't move to vinca master without regenerating their snapshot, which also bumps hundreds of package versions and forces rebuilds.

This PR makes that case a fallback instead of an error:

• Snapshots without dependency metadata: _get_direct_depends resolves dependencies from the rosdistro cache through the DependencyWalker. That's the same source vinca-snapshot uses to fill the field.
• Warning: vinca prints a one-time warning saying the snapshot should be regenerated to pin its dependencies.
• Snapshots with dependencies: unchanged.

Testing

• pytest vinca/: 251 passed. test_snapshot_without_dependencies_requires_regeneration is replaced by a test of the fallback.
• ros-rolling main on Linux (linux-64), with its current snapshot and skip_existing removed (to generate every recipe):
  • With this branch: 1927 recipes, the same set as ros-rolling's current vinca pin (pre-snapshot commit + fix: use explicit refs/tags/ in GitHub raw package.xml URLs #156), minus system_modes/system_modes_msgs. Those are no longer in rolling's rosdistro, so the cache can't resolve them.
  • With vinca master: fails on the first package.
• CI: a demo of rolling on this vinca passes on all five platforms: Demo: ros-rolling instantiated from ros-distro-template Tobias-Fischer/ros-rolling#1.

🤖 Generated with Claude Code

[traversaro]

Thanks!