Add "default" configuration scope.

- Default scope is versioned with spack and can be overridden by site
  or user config.

- Default scope provides sensible default concretization preferences
  for all of Spack.

- per-platform concretization scope can be added later (to force a
  particular MPI on, e.g., Cray systems)

etc/spack/defaults/modules.yaml

@@ -1,8 +1,17 @@
 # -------------------------------------------------------------------------
-# This is the default spack module files generation configuration.
+# This is the default configuration for Spack's module file generation.
 #
-# Changes to this file will affect all users of this spack install,
-# although users can override these settings in their ~/.spack/modules.yaml.
+# 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/modules.yaml
+#
+# Per-user settings (overrides default and site settings):
+#   ~/.spack/modules.yaml
 # -------------------------------------------------------------------------
 modules:
   enable:

etc/spack/defaults/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:
+    providers:
+      mpi: [openmpi, mpich]
+      blas: [openblas]
+      lapack: [openblas]

etc/spack/defaults/repos.yaml

@@ -0,0 +1,14 @@
+# -------------------------------------------------------------------------
+# This is the default spack repository configuration. It includes the
+# builtin spack package repository.
+#
+# Users can override these settings by editing the following files.
+#
+# Per-spack-instance settings (overrides defaults):
+#   $SPACK_ROOT/etc/spack/repos.yaml
+#
+# Per-user settings (overrides default and site settings):
+#   ~/.spack/repos.yaml
+# -------------------------------------------------------------------------
+repos:
+  - $spack/var/spack/repos/builtin

etc/spack/repos.yaml

@@ -1,8 +0,0 @@
-# -------------------------------------------------------------------------
-# This is the default spack repository configuration.
-#
-# Changes to this file will affect all users of this spack install,
-# although users can override these settings in their ~/.spack/repos.yaml.
-# -------------------------------------------------------------------------
-repos:
-  - $spack/var/spack/repos/builtin