fix: override mgs

[matt-FFFFFF]

Fix bug where override archetypes had to be in same library instance as the base archetype.

Also:

• improved modify policy assignments func to support overrides and resource selectors

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 73.91304% with 6 lines in your changes are missing coverage. Please review.

Project coverage is 75.25%. Comparing base (106b88f) to head (80f60d4). Report is 52 commits behind head on main.

Files	Patch %	Lines
alzlib.go	64.70%	3 Missing and 3 partials :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #85 +/- ## ========================================== + Coverage 70.66% 75.25% +4.59% ========================================== Files 6 6 Lines 900 772 -128 ========================================== - Hits 636 581 -55 + Misses 193 117 -76 - Partials 71 74 +3 ```

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