Fix joint parent/child frame existence checks to include interface elements

[azeey]

🦟 Bug fix

Summary

Names in //joint/parent or //joint/child that reference frames inside a nested model that was included via the Interface API cause errors when loading a model. I believe this is a regression that was introduced when fixing #719 via #727.

9b8a2bd adds a failing test showing the regression with the following failure message:

Value of: errors.empty()
  Actual: false
Expected: true
Error Code 19: Msg: child frame with name[double_pendulum::base] specified by joint with name[J1] not found in model with name[parent_model].
Error Code 20: Msg: parent frame with name[double_pendulum::child_dp::base] specified by joint with name[J2] not found in model with name[parent_model].

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)
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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-2022-03-25-fortress-edifice-citadel/1343/1

[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