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.
7ffe2fadfe