[llvm] [mlir] [MLIR][Python] use `FetchContent_Declare` for nanobind and remove pybind (PR #161230)

[Konrad Kleine via llvm-commits]

kwk wrote:

Can you please change the logic to be `find_package` first and only if it doesn't find nanobind, you fetch use `FetchContent`? Simply get rid of the `REQUIRED` option to `find_package`.

```
cmake_minimum_required(VERSION 3.31)

project(myproject)

find_package(Python COMPONENTS Interpreter Development REQUIRED)

# See this for how to find nanobind:
# https://nanobind.readthedocs.io/en/latest/building.html#finding-nanobind
execute_process(
  COMMAND "${Python_EXECUTABLE}" -m nanobind --cmake_dir
  OUTPUT_STRIP_TRAILING_WHITESPACE OUTPUT_VARIABLE nanobind_ROOT)

find_package(nanobind 2.9 CONFIG)

message(STATUS "nanobind_DIR: ${nanobind_DIR}")

if (${nanobind_DIR} STREQUAL "nanobind_DIR-NOTFOUND")
    message(STATUS "nanobind not found Use FetchContent")
else()
    message(STATUS "nanobind found")
endif()
```

https://github.com/llvm/llvm-project/pull/161230