Define Python_FIND_VIRTUALENV in addition to Python3_FIND_VIRTUALENV

[dbabokin]

MLIR searches for both Python3 and Python. If both variables are not defined equally and there multiple python versions on the system, CMake may behave really weirdly.

More details here: llvm/llvm-project#126162

So I grepped for all instances of Python3_FIND_VIRTUALENV and added similar Python_FIND_VIRTUALENV definition.

[dbabokin]

The fails in the CI are unrelated to the changes in this PR and they show up in other PR as well.

[dbabokin]

All green. Need someone to review and approve.

[marbre]

Thanks @dbabokin! Can you resolve the conflicts thus we can merge this? I guess we could also set the one option based on the other a little deeper in CMake world? But this also works for me.

[ScottTodd]

MLIR searches for both Python3 and Python. If both variables are not defined equally and there multiple python versions on the system, CMake may behave really weirdly.

Could MLIR use IREE's approach instead? https://github.com/iree-org/iree/blob/fd74b8829bebb9c64d45d265d906dbc7b0301c40/CMakeLists.txt#L654-L663

if(IREE_BUILD_PYTHON_BINDINGS)
  # ...
  find_package(Python3 3.9 COMPONENTS Interpreter Development NumPy)
  find_package(Python3 3.9 COMPONENTS Interpreter Development.Module NumPy REQUIRED)
  # Some parts of the build use FindPython instead of FindPython3. Why? No
  # one knows, but they are different. So make sure to bootstrap this one too.
  # Not doing this here risks them diverging, which on multi-Python systems,
  # can be troublesome. Note that nanobind requires FindPython.
  set(Python_EXECUTABLE "${Python3_EXECUTABLE}")
  find_package(Python 3.9 COMPONENTS Interpreter Development.Module NumPy REQUIRED)
elseif(IREE_BUILD_COMPILER OR IREE_BUILD_TESTS)
  find_package(Python3 COMPONENTS Interpreter REQUIRED)
  set(Python_EXECUTABLE "${Python3_EXECUTABLE}")
  find_package(Python COMPONENTS Interpreter REQUIRED)
endif()

That's been working well for us, and it's less configuration needed by the user. One small bandaid in the build system is fine with me... but this starts to spread extra arguments across scripts and documentation too.