Improve reusability in colcon_ros.package_identification.ros

[cottsay]

• Expose public functions for performing package augmentation
• Ensure dependency categories are set on descriptors even when those categories are empty (to align with colcon_core)

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 62.06897% with 11 lines in your changes are missing coverage. Please review.

Project coverage is 15.82%. Comparing base (f234777) to head (4ab4d49).

Files	Patch %	Lines
colcon_ros/package_identification/ros.py	62.06%	8 Missing and 3 partials :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #160 +/- ## ========================================== + Coverage 15.53% 15.82% +0.28% ========================================== Files 20 20 Lines 650 651 +1 Branches 139 135 -4 ========================================== + Hits 101 103 +2 - Misses 539 540 +1 + Partials 10 8 -2 ```

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