Windows: package defaults and MPI detection (#34614)

* Update packages config to indicate that MSVC is the preferred compiler
* Update packages config to indicate that msmpi is the preferred MPI provider
* Fix msmpi external detection

etc/spack/defaults/windows/packages.yaml

@@ -0,0 +1,21 @@
+# -------------------------------------------------------------------------
+# This file controls default concretization preferences for Spack.
+#
+# Settings here are versioned with Spack and are intended to provide
+# sensible defaults out of the box. Spack maintainers should edit this
+# file to keep it current.
+#
+# Users can override these settings by editing the following files.
+#
+# Per-spack-instance settings (overrides defaults):
+#   $SPACK_ROOT/etc/spack/packages.yaml
+#
+# Per-user settings (overrides default and site settings):
+#   ~/.spack/packages.yaml
+# -------------------------------------------------------------------------
+packages:
+  all:
+    compiler:
+    - msvc
+    providers:
+      mpi: [msmpi]