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]

var/spack/repos/builtin/packages/wgl/package.py

@@ -34,6 +34,7 @@ class Wgl(Package):
     version("10.0.14393")
     version("10.0.10586")
     version("10.0.26639")
+    version("10.0.20348")
 
     # As per https://github.com/spack/spack/pull/31748 this provisory version represents
     # an arbitrary openGL version designed for maximum compatibility with calling packages
@@ -42,12 +43,12 @@ class Wgl(Package):
     # satisfied appropriately
     provides("gl@4.6")
 
-    variant("plat", values=("x64", "x86", "arm", "arm64"), default="x64")
-
     # WGL exists on all Windows systems post win 98, however the headers
     # needed to use OpenGL are found in the SDK (GL/gl.h)
     # Dep is needed to consolidate sdk version to locate header files for
     # version of SDK being used
+    # Generic depends to capture handling for external versions
+    depends_on("win-sdk")
     depends_on("win-sdk@10.0.19041", when="@10.0.19041")
     depends_on("win-sdk@10.0.18362", when="@10.0.18362")
     depends_on("win-sdk@10.0.17763", when="@10.0.17763")
@@ -77,14 +78,32 @@ def determine_variants(cls, libs, ver_str):
             variants.append("plat=%s" % arch)
         return variants
 
+    def _spec_arch_to_sdk_arch(self):
+        spec_arch = str(self.spec.architecture.target).lower()
+        _64bit = "64" in spec_arch
+        arm = "arm" in spec_arch
+        if arm:
+            return "arm64" if _64bit else "arm"
+        else:
+            return "x64" if _64bit else "x86"
+
     # As noted above, the headers neccesary to include
     @property
     def headers(self):
-        return find_headers("GL/gl.h", root=self.spec["win-sdk"].prefix.includes, recursive=True)
+        return find_headers(
+            "GL", root=os.path.join(self.prefix.Include, str(self.version) + ".0"), recursive=True
+        )
 
     @property
     def libs(self):
-        return find_libraries("opengl32", shared=False, root=self.prefix, recursive=True)
+        return find_libraries(
+            "opengl32",
+            shared=False,
+            root=os.path.join(
+                self.prefix.Lib, str(self.version) + ".0", "um", self._spec_arch_to_sdk_arch()
+            ),
+            recursive=True,
+        )
 
     def install(self, spec, prefix):
         raise RuntimeError(