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[bot]]

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 :warning:
colcon_core/shell/__init__.py	75.00%	0 Missing and 2 partials :warning:
colcon_core/argument_parser/__init__.py	75.00%	0 Missing and 1 partial :warning:
colcon_core/environment/__init__.py	75.00%	0 Missing and 1 partial :warning:
colcon_core/event_handler/__init__.py	75.00%	0 Missing and 1 partial :warning:
colcon_core/executor/__init__.py	75.00%	0 Missing and 1 partial :warning:
colcon_core/package_augmentation/__init__.py	75.00%	0 Missing and 1 partial :warning:
colcon_core/package_discovery/__init__.py	75.00%	0 Missing and 1 partial :warning:
colcon_core/package_identification/__init__.py	75.00%	0 Missing and 1 partial :warning:
colcon_core/package_selection/__init__.py	75.00%	0 Missing and 1 partial :warning:
... and 2 more

Additional details and impacted files

```diff @@ 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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