Set default provider for every virtual package (#7784)

* Set default provider for every virtual package

* Add unit test to ensure that every virtual package has a default provider explicitly set

lib/spack/spack/test/package_sanity.py

@@ -60,3 +60,13 @@ def test_all_versions_are_lowercase():
             errors.append(name)
 
     assert len(errors) == 0
+
+
+def test_all_virtual_packages_have_default_providers():
+    """All virtual packages must have a default provider explicitly set."""
+    defaults = spack.config.get_config('packages', scope='defaults')
+    default_providers = defaults['all']['providers']
+    providers = spack.repo.provider_index.providers
+
+    for provider in providers:
+        assert provider in default_providers