WGL package: correct libs/headers detection (#35113)

Corrects libs detection with a more specific root, otherwise there
can be inconsistencies between version of WGL requested and the
version picked up by `find_libraries`.

Corrects headers detection - win-sdk, win-wdk, and WGL headers all
exist under the same directory, so we can compute the headers for WGL
without querying the spec for win-sdk (which causes errors).

This commit also removes the `plat` variant of `wgl`, which is
redundant with the Spec's target.

etc/spack/defaults/windows/packages.yaml

@@ -19,3 +19,4 @@ packages:
     - msvc
     providers:
       mpi: [msmpi]
+      gl: [wgl]