Handle __model__ in joint parent or child when using merge-include

[azeey]

🦟 Bug fix

Summary

With #833 allowed using __model__ in //joint/parent or //joint/child. When merge-including models that include such cases, the __model__ has to be changed to the proxy frame.

Checklist

• Signed all commits for DCO
• Added tests
• [ ] Updated documentation (as needed)
• [ ] Updated migration guide (as needed)
• codecheck passed (See contributing)
• All tests passed (See test coverage)
• While waiting for a review on your PR, please help review another open pull request to support the maintainers

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[codecov-commenter]

Codecov Report

Merging #835 (1f1daea) into sdf12 (0351a39) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##            sdf12     #835   +/-   ##
=======================================
  Coverage   90.70%   90.71%           
=======================================
  Files          78       78           
  Lines       12441    12447    +6     
=======================================
+ Hits        11285    11291    +6     
  Misses       1156     1156           

Impacted Files	Coverage Δ
src/parser.cc	87.58% <100.00%> (+0.06%)	⬆️

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[osrf-triage]

This pull request has been mentioned on Gazebo Community. There might be relevant details there:

https://community.gazebosim.org/t/new-ignition-releases-2022-04-13-fortress-edifice/1367/1