Basic check for winsdk before concretizing

[johnwparent]

Adds a pre-concretization check in asp.py for the Windows SDK and WGL (Windows GL) packages as non-buildable externals.
Previously this was incorporated into ensure_core_depenendencies in the bootstrap module, but that scoped the config modifications to the bootstrap scope, which made the changes fundamentally useless for general purpose use, so we transitioned to a pre concretization call to ensure these packages are always available pre concretization.

The goal is to achieve parity with most unix-like systems where we have a consistent understanding of the system (potentially hardware accelerated) GL location and can reasonably rely on that heuristic to bake the external into packages.yaml.
Adds documentation to the startup guide denoting that the SDK is a hard requirement for Spack now.
Adds error handling if WGL and WSDK cannot be detected on system.

Adds pytest fixture mocking the check for WGL and WSDK as if they were present

Note: This fixture requires the session scope in order to avoid a race where certain fixtures are invoked first without the mocked WSDK check. Typical pytest monkeypatch fixtures are function scope, which is incompatible with session scoped fixtures. This required the introduction of a session scoped monkeypatch fixture named monkeypatch_session.

[scheibelp]

I have a couple minor requests about documentation. This generally looks good.

[johnwparent]

@spackbot fix style

[spackbot-app]

Let me see if I can fix that for you!

[spackbot-app]

I was able to run spack style --fix for you!

spack style --fix

==> Running style checks on spack
  selected: isort, black, flake8, mypy
==> Modified files
  lib/spack/spack/bootstrap/core.py
  lib/spack/spack/cmd/unit_test.py
  lib/spack/spack/solver/asp.py
  lib/spack/spack/test/conftest.py
==> Running isort checks
  isort checks were clean
==> Running black checks
reformatted lib/spack/spack/solver/asp.py
All done! ✨ 🍰 ✨
1 file reformatted, 3 files left unchanged.
  black checks were clean
==> Running flake8 checks
  flake8 checks were clean
==> Running mypy checks
lib/spack/spack/version/version_types.py:145: error: Argument 2 to "StandardVersion" has incompatible type "*Tuple[Tuple[Any, ...], Tuple[Any, ...]]"; expected "Tuple[Tuple[Any, ...], Tuple[Any, ...]]"  [arg-type]
lib/spack/spack/version/version_types.py:452: error: Argument 2 to "StandardVersion" has incompatible type "*Tuple[Tuple[Any, ...], Tuple[Any, ...]]"; expected "Tuple[Tuple[Any, ...], Tuple[Any, ...]]"  [arg-type]
lib/spack/spack/version/version_types.py:481: error: Argument 2 to "StandardVersion" has incompatible type "*Tuple[Tuple[Any, ...], Tuple[Any, ...]]"; expected "Tuple[Tuple[Any, ...], Tuple[Any, ...]]"  [arg-type]
Found 3 errors in 1 file (checked 623 source files)
  mypy found errors

Keep in mind that I cannot fix your flake8 or mypy errors, so if you have any you'll need to fix them and update the pull request. If I was able to push to your branch, if you make further changes you will need to pull from your updated branch before pushing again.

I've updated the branch with style fixes.