Support alternate group names in get_*_extensions

[cottsay]

These functions are brief, but it would be nice not to have to duplicate them in other colcon packages which re-use the same extension frameworks.

[cottsay]

Example of such duplication: https://github.com/colcon/colcon-installed-package-information/blob/18a770ee6e6f07d06300568660e9c6b62d17d4b7/colcon_installed_package_information/package_identification/__init__.py#L8

[codecov]

Codecov Report

Attention: Patch coverage is 71.15385% with 15 lines in your changes are missing coverage. Please review.

Project coverage is 83.18%. Comparing base (b174608) to head (fe660e8).
Report is 1 commits behind head on master.

Files	Patch %	Lines
colcon_core/task/python/test/__init__.py	25.00%	3 Missing ⚠️
colcon_core/shell/__init__.py	75.00%	0 Missing and 2 partials ⚠️
colcon_core/argument_parser/__init__.py	75.00%	0 Missing and 1 partial ⚠️
colcon_core/environment/__init__.py	75.00%	0 Missing and 1 partial ⚠️
colcon_core/event_handler/__init__.py	75.00%	0 Missing and 1 partial ⚠️
colcon_core/executor/__init__.py	75.00%	0 Missing and 1 partial ⚠️
colcon_core/package_augmentation/__init__.py	75.00%	0 Missing and 1 partial ⚠️
colcon_core/package_discovery/__init__.py	75.00%	0 Missing and 1 partial ⚠️
colcon_core/package_identification/__init__.py	75.00%	0 Missing and 1 partial ⚠️
colcon_core/package_selection/__init__.py	75.00%	0 Missing and 1 partial ⚠️
... and 2 more

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #647      +/-   ##
==========================================
- Coverage   83.43%   83.18%   -0.26%     
==========================================
  Files          66       66              
  Lines        3816     3842      +26     
  Branches      745      758      +13     
==========================================
+ Hits         3184     3196      +12     
- Misses        555      557       +2     
- Partials       77       89      +12     

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