[mlir] Refactor finding python

[marbre]

Summary

This drops the use of deprecated CMake modules to find python.

[marbre]

LLVM already relies on find_package(Python3). Prior to the patch different Python versions are found on a system with multiple versions installed (here 3.9 and 3.7):

-- Found Python3: /usr/bin/python3.9 (found version "3.9.0") found components: Interpreter 
-- Constructing LLVMBuild project information
-- Linker detection: LLD
-- Found Git: /usr/bin/git (found version "2.26.2") 
-- Targeting X86
-- Performing Test C_SUPPORTS_WERROR_IMPLICIT_FUNCTION_DECLARATION
-- Performing Test C_SUPPORTS_WERROR_IMPLICIT_FUNCTION_DECLARATION - Success
-- Found PythonInterp: /usr/bin/python (found version "3.7.9") 
-- Found PythonLibs: /usr/lib64/libpython3.7m.so (found version "3.7.9") 
-- Found python include dirs: /usr/include/python3.7m
-- Found python libraries: /usr/lib64/libpython3.7m.so
-- Found PythonLibs: /usr/lib64/libpython3.7m.so
-- Found pybind11 v2.5.0: /usr/include;/usr/include;/usr/include/python3.7m
-- Python prefix = '', suffix = '', extension = '.cpython-37m-x86_64-linux-gnu.so

This is fixed with the proposed patch:

-- Found Python3: /usr/bin/python3.9 (found version "3.9.0") found components: Interpreter 
-- Constructing LLVMBuild project information
-- Linker detection: LLD
-- Found Git: /usr/bin/git (found version "2.26.2") 
-- Targeting X86
-- Performing Test C_SUPPORTS_WERROR_IMPLICIT_FUNCTION_DECLARATION
-- Performing Test C_SUPPORTS_WERROR_IMPLICIT_FUNCTION_DECLARATION - Success
-- Found Python3: /usr/bin/python3.9 (found version "3.9.0") found components: Interpreter Development 
-- Found python include dirs: /usr/include/python3.9
-- Found python libraries: /usr/lib64/libpython3.9.so
-- Found PythonInterp: /usr/bin/python3.9 (found version "3.9") 
-- Found PythonLibs: /usr/lib64/libpython3.9.so
-- Found pybind11 v2.5.0: /usr/include;/usr/include;/usr/include/python3.9
-- Python prefix = '', suffix = '', extension = '.cpython-39-x86_64-linux-gnu.so

mlir/CMakeLists.txt

 #-------------------------------------------------------------------------------
 
 set(MLIR_BINDINGS_PYTHON_ENABLED 0 CACHE BOOL
        "Enables building of Python bindings.")
 set(MLIR_PYTHON_BINDINGS_VERSION_LOCKED 1 CACHE BOOL
        "Links to specific python libraries, resolving all symbols.")
 
 if(MLIR_BINDINGS_PYTHON_ENABLED)
-  find_package(PythonInterp REQUIRED)
-  find_package(PythonLibs REQUIRED)
-  message(STATUS "Found python include dirs: ${PYTHON_INCLUDE_DIRS}")
-  message(STATUS "Found python libraries: ${PYTHON_LIBRARIES}")
+  find_package(Python3 COMPONENTS Interpreter Development REQUIRED)
+  message(STATUS "Found python include dirs: ${Python3_INCLUDE_DIRS}")
+  message(STATUS "Found python libraries: ${Python3_LIBRARIES}")
   find_package(pybind11 CONFIG REQUIRED)
   # TODO: pybind11 v2.6 switched from pybind11_INCLUDE_DIRS (plural) to
   # pybind11_INCLUDE_DIR (singular). A lot has changed in this area since this
   # was written and overall python config and pybind11 should be modernized.
   set(pybind11_INCLUDE_DIR ${pybind11_INCLUDE_DIR} ${pybind11_INCLUDE_DIRS})
   message(STATUS "Found pybind11 v${pybind11_VERSION}: ${pybind11_INCLUDE_DIR}")
   message(STATUS "Python prefix = '${PYTHON_MODULE_PREFIX}', "
                  "suffix = '${PYTHON_MODULE_SUFFIX}', "

mlir/cmake/modules/AddMLIRPythonExtension.cmake

function(add_mlir_python_extension libname extname)
   # some greater deployment flexibility since the extension will bind to
   # symbols in the python interpreter on load. However, it also keeps the
   # linker from erroring on undefined symbols, leaving this to (usually obtuse)
   # runtime errors. Building in "SHARED" mode with an explicit link to the
   # python libraries allows us to build with the expectation of no undefined
   # symbols, which is better for development. Note that not all python
   # configurations provide build-time libraries to link against, in which
   # case, we fall back to MODULE linking.
-  if(PYTHON_LIBRARIES STREQUAL "" OR NOT MLIR_PYTHON_BINDINGS_VERSION_LOCKED)
+  if(Python3_LIBRARIES STREQUAL "" OR NOT MLIR_PYTHON_BINDINGS_VERSION_LOCKED)
     set(PYEXT_LINK_MODE MODULE)
     set(PYEXT_LIBADD)
   else()
     set(PYEXT_LINK_MODE SHARED)
-    set(PYEXT_LIBADD ${PYTHON_LIBRARIES})
+    set(PYEXT_LIBADD ${Python3_LIBRARIES})
   endif()
 
   # The actual extension library produces a shared-object or DLL and has
   # sources that must be compiled in accordance with pybind11 needs (RTTI and
   # exceptions).
   add_library(${libname} ${PYEXT_LINK_MODE}
     ${ARG_SOURCES}
   )
 
   target_include_directories(${libname} PRIVATE
-    "${PYTHON_INCLUDE_DIRS}"
+    "${Python3_INCLUDE_DIRS}"
     "${pybind11_INCLUDE_DIR}"
   )
 
   # The extension itself must be compiled with RTTI and exceptions enabled.
   # Also, some warning classes triggered by pybind11 are disabled.
   target_compile_options(${libname} PRIVATE
     $<$<OR:$<CXX_COMPILER_ID:Clang>,$<CXX_COMPILER_ID:AppleClang>,$<CXX_COMPILER_ID:GNU>>:
       # Enable RTTI and exceptions.