Replace FetchContent with a custom dependency provider

[alexreinking]

This is the first big change from #8360

The build no longer uses FetchContent, instead using find_package always and everywhere. When Halide is the top-level project, it will (by default) inject a dependency provider that overrides the wabt, flatbuffers, and pybind11 packages with FetchContent. Users can opt-out by setting Halide_USE_FETCHCONTENT=NO.

This also bumps the required wabt version to the latest release (1.0.36). This version includes a patch I submitted that fixes the CMake package when wabt is built with OpenSSL rather than picosha2.

---

Here are relevant links to the docs:

• https://cmake.org/cmake/help/latest/guide/using-dependencies/index.html#dependency-providers
• https://cmake.org/cmake/help/latest/command/cmake_language.html#dependency-providers

[alexreinking]

Spurious failure in performance_inner_loop_parallel on different targets, both on linux-worker-1.