Rework WIL dependency retrieval/usage (#17130)
### Description
1. `onnxruntime_fetchcontent_makeavailable` works around unconditional
install commands so that can be used instead of `FetchContent_Populate`
2. This dependency is Windows specific, mark it as such.
### Motivation and Context
1. This simplifies `cmake/external/wil.cmake` not to do anything
specific wether WIL was fetched or found
2. Given it's specific to Windows, it might not be available on other OS
in specific air-gapped environment such as
[conan-center-index](https://github.com/conan-io/conan-center-index).
This allows downstream builds not to require specific patches for
something not required by the build in the first place.
