distro: make it impossible to initialize registry with nil values

The current `NewRegistry` implementation allows for nil values in the
map, but this leads to subtle bugs when using the registry. This patch
enforces non-nil values by introducing additional checks before we
insert the value into the map.

The change unfortunately breaks a lot of tests and therefore it is
necessary to create additional mock: distro.

The new mock is used instead of the previous "real" implementation,
which used to contain nil values.

internal/mocks/distro/distro_mock.go

@@ -0,0 +1,14 @@
+package distro_mock
+
+import (
+	"github.com/osbuild/osbuild-composer/internal/distro"
+	"github.com/osbuild/osbuild-composer/internal/distro/fedoratest"
+)
+
+func NewRegistry() *distro.Registry {
+	ftest := fedoratest.New()
+	if ftest == nil {
+		panic("Attempt to register Fedora test failed")
+	}
+	return distro.WithSingleDistro(ftest)
+}

internal/mocks/rpmmd/fixtures.go

@@ -4,7 +4,7 @@ import (
 	"fmt"
 	"github.com/osbuild/osbuild-composer/internal/common"
 	"github.com/osbuild/osbuild-composer/internal/compose"
-	"github.com/osbuild/osbuild-composer/internal/distro"
+	distro_mock "github.com/osbuild/osbuild-composer/internal/mocks/distro"
 	"sort"
 	"time"
 
@@ -94,7 +94,7 @@ func createBaseStoreFixture() *store.Store {
 	}
 
 	d := test_distro.New()
-	r := distro.NewRegistry([]string{"."})
+	r := distro_mock.NewRegistry()
 	s := store.New(nil, d, *r)
 
 	s.Blueprints[bName] = b
@@ -184,7 +184,7 @@ func createStoreWithoutComposesFixture() *store.Store {
 	}
 
 	d := test_distro.New()
-	r := distro.NewRegistry([]string{"."})
+	r := distro_mock.NewRegistry()
 	s := store.New(nil, d, *r)
 
 	s.Blueprints[bName] = b