Windows: auto-add WGL/SDK as externals (#43752)

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.

lib/spack/docs/getting_started.rst

@@ -1572,6 +1572,8 @@ Microsoft Visual Studio
 """""""""""""""""""""""
 
 Microsoft Visual Studio provides the only Windows C/C++ compiler that is currently supported by Spack.
+Spack additionally requires that the Windows SDK (including WGL) to be installed as part of your
+visual studio installation as it is required to build many packages from source.
 
 We require several specific components to be included in the Visual Studio installation.
 One is the C/C++ toolset, which can be selected as "Desktop development with C++" or "C++ build tools,"
@@ -1579,6 +1581,7 @@ depending on installation type (Professional, Build Tools, etc.)  The other requ
 "C++ CMake tools for Windows," which can be selected from among the optional packages.
 This provides CMake and Ninja for use during Spack configuration.
 
+
 If you already have Visual Studio installed, you can make sure these components are installed by
 rerunning the installer.  Next to your installation, select "Modify" and look at the
 "Installation details" pane on the right.