cmd/schemagen: refactoring

- Replace log.Faltal with error wrapping in schemagen func
- Simplify tests, use temp dir and ioutil functions, remove boilerplate code
- In test use schemagen keyspace to avoid name conflict with examples
- Change template

cmd/schemagen/testdata/models.go.txt

@@ -1,42 +1,43 @@
 // Code generated by "gocqlx/cmd/schemagen"; DO NOT EDIT.
 
-package {{pkgname}}
+package foobar
 
 import "github.com/scylladb/gocqlx/v2/table"
 
-var PlaylistsMetadata = table.Metadata{
-	Name: "playlists",
-	Columns: []string{
-		"album",
-		"artist",
-		"id",
-		"song_id",
-		"title",
-	},
-	PartKey: []string{
-		"id",
-	},
-	SortKey: []string{
-		"title",
-		"album",
-		"artist",
-	},
-}
-var PlaylistsTable = table.New(PlaylistsMetadata)
+// Table models.
+var (
+	Playlists = table.New(table.Metadata{
+		Name: "playlists",
+		Columns: []string{
+			"album",
+			"artist",
+			"id",
+			"song_id",
+			"title",
+		},
+		PartKey: []string{
+			"id",
+		},
+		SortKey: []string{
+			"title",
+			"album",
+			"artist",
+		},
+	})
 
-var SongsMetadata = table.Metadata{
-	Name: "songs",
-	Columns: []string{
-		"album",
-		"artist",
-		"data",
-		"id",
-		"tags",
-		"title",
-	},
-	PartKey: []string{
-		"id",
-	},
-	SortKey: []string{},
-}
-var SongsTable = table.New(SongsMetadata)
+	Songs = table.New(table.Metadata{
+		Name: "songs",
+		Columns: []string{
+			"album",
+			"artist",
+			"data",
+			"id",
+			"tags",
+			"title",
+		},
+		PartKey: []string{
+			"id",
+		},
+		SortKey: []string{},
+	})
+)