gz-msgs-extras: Do not call find_package(Python3) if gz-msgs<majVer>_PYTHON_INTERPRETER is defined

[traversaro]

🦟 Bug fix

Do not raise error in cross-compilation context if gz-msgs<majorVer>__PYTHON_INTERPRETER is defined.

Summary

When gz-msgs<majorVer>__PYTHON_INTERPRETER is defined, its value will be used instead of Python3::Interpreter. For this reason, it does not make sense to search for Python3 if gz-msgs<majorVer>__PYTHON_INTERPRETER is defined. Moving the find_package there simplifies the cross-compilation of gz packages, as it remove the need for CMake to find a working Python interpeter.

It would be cool (regardless of the value of gz-msgs<majorVer>__PYTHON_INTERPRETER) to only search for Python3 if one project actually calls gz_msgs_generate_messages, and not if a project just call (sometimes transitively) find_package(gz-msgs<majorVer>). However, this would be a more complex change, while the change of this PR is simple and self-contained.

Checklist

• Signed all commits for DCO
• Added tests
• Updated documentation (as needed)
• Updated migration guide (as needed)
• Consider updating Python bindings (if the library has them)
• 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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[traversaro]

It would be cool (regardless of the value of gz-msgs<majorVer>__PYTHON_INTERPRETER) to only search for Python3 if one project actually calls gz_msgs_generate_messages, and not if a project just call (sometimes transitively) find_package(gz-msgs<majorVer>). However, this would be a more complex change, while the change of this PR is simple and self-contained.

Actually I think this is doable, let me close this and open a separate PR for that.