Adds a pre-concretization check for the Windows SDK and WGL (Windows
GL) packages as non-buildable externals.
This is a redo of https://github.com/spack/spack/pull/43459, but makes
sure to modify the configuration scope outside of the bootstrap scope:
whichever is highest-precedence in the user's environment at the time
the concretization runs, which should either be an env scope or the
~ scope.
Adds pytest fixture mocking the check for WGL and WSDK as if they were
present.
7e8415a3a6