cmake extras: only find Python3 if needed

[scpeters]

🦟 Bug fix

Fixes gz-transport python bindings

Summary

If Python3 has already been found, we shouldn't search again since it may find different components.

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

Note to maintainers: Remember to use Squash-Merge and edit the commit message to match the pull request summary while retaining Signed-off-by messages.

[scpeters]

testing gz-transport14 with this PR in gazebosim/gz-transport#566

checking that python bindings are built in https://build.osrfoundation.org/job/gz_transport-ci-pr_any-homebrew-amd64/217/

[scpeters]

testing gz-transport14 with this PR in gazebosim/gz-transport#566

checking that python bindings are built in https://build.osrfoundation.org/job/gz_transport-ci-pr_any-homebrew-amd64/217/

I see python tests were run successfully in that build, so I think this fixes the issue

[azeey]

LGTM!

I ran the following and confirmed that Python3_Development_FOUND is set to false after the second call to find_package.

cmake_minimum_required(VERSION 3.5)
project(test_cmake)
include(CMakePrintHelpers)

find_package(Python3 REQUIRED COMPONENTS Development Interpreter)
cmake_print_variables(Python3_Development_FOUND)
cmake_print_variables(Python3_Interpreter_FOUND)
message("Now calling find_package again")
find_package(Python3 REQUIRED COMPONENTS Interpreter)
cmake_print_variables(Python3_Development_FOUND)
cmake_print_variables(Python3_Interpreter_FOUND)

[scpeters]

LGTM!

I ran the following and confirmed that Python3_Development_FOUND is set to false after the second call to find_package.

thanks! I meant to clarify what I saw in the gz-transport build logs that made me suspect this was the issue. I'll post it here for posterity.

From https://build.osrfoundation.org/job/gz_transport-ci-gz-transport14-noble-amd64/45/:

-- ====== Finding Dependencies ======
-- Found Python3: /usr/bin/python3 (found version "3.12.3") found components: Interpreter Development Development.Module Development.Embed 
-- Found Protobuf: /usr/lib/x86_64-linux-gnu/libprotobuf.so (found version "3.21.12") 
...
-- Looking for gz-msgs11 -- found version 11.0.1
-- Searching for dependencies of gz-msgs11
-- Looking for gz-utils3 -- found version 3.1.0
-- Looking for gz-math8 -- found version 8.0.0
-- Searching for dependencies of gz-math8
-- Looking for gz-utils3 -- found version 3.1.0
-- Checking for module 'tinyxml2'
--   Found tinyxml2, version 10.0.0
-- Found Python3: /usr/bin/python3 (found version "3.12.3") found components: Interpreter 
-- Looking for gz-msgs11 - found

that second Found Python3 message occurs while finding gz-msgs, which is what pointed me in this direction

in the test build using this branch, I see the following when finding gz-msgs:

-- Looking for gz-msgs11 -- found version 11.0.1
-- Searching for dependencies of gz-msgs11
-- Looking for gz-utils3 -- found version 3.1.0
-- Looking for gz-math8 -- found version 8.1.0
-- Searching for dependencies of gz-math8
-- Looking for gz-utils3 -- found version 3.1.0
-- Checking for module 'tinyxml2'
--   Found tinyxml2, version 10.0.0
-- Looking for gz-msgs11 - found