DM-45767: Move CollectionType definition up a level

[timj]

Checklist

• ran Jenkins
• added a release note for user-visible changes to doc/changes
• (if changing dimensions.yaml) make a copy of dimensions.yaml in configs/old_dimensions

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 89.51%. Comparing base (b716002) to head (0885e05).
Report is 3 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff            @@
##             main    #1055    +/-   ##
========================================
  Coverage   89.51%   89.51%            
========================================
  Files         361      361            
  Lines       46137    46147    +10     
  Branches     9460     8819   -641     
========================================
+ Hits        41299    41308     +9     
  Misses       3504     3504            
- Partials     1334     1335     +1     

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[andy-slac]

Looks great. This probably needs a news fragment.