refactor(generator): split ProtoDefinitionLocation

[coryan]

Part of the work for #7664

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This change is 

[codecov]

Codecov Report

Base: 93.73% // Head: 93.73% // Increases project coverage by +0.00% 🎉

Coverage data is based on head (b823e8e) compared to base (e773e4e).
Patch coverage: 100.00% of modified lines in pull request are covered.

Additional details and impacted files

@@           Coverage Diff           @@
##             main   #10674   +/-   ##
=======================================
  Coverage   93.73%   93.73%           
=======================================
  Files        1688     1688           
  Lines      152699   152699           
=======================================
+ Hits       143135   143137    +2     
+ Misses       9564     9562    -2     

Impacted Files	Coverage Δ
generator/internal/resolve_comment_references.cc	97.36% <100.00%> (ø)
...erator/internal/resolve_comment_references_test.cc	86.79% <100.00%> (ø)
google/cloud/pubsub/subscriber_connection_test.cc	100.00% <0.00%> (+1.21%)	⬆️
...e/cloud/pubsublite/internal/alarm_registry_impl.cc	100.00% <0.00%> (+2.94%)	⬆️

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