env: Move each component into a separate struct

This commit cleans up the distillery code by making each component a
distinct struct. Each of these components is also returned by by a new
Component() function that replaces the Stacks() function.

cmd/make_mysql_account.go

@@ -55,7 +55,7 @@ func (mma makeMysqlAccount) Run(context wisski_distillery.Context) error {
 	if err != nil {
 		return err
 	}
-	code, err := context.Environment.SQLShell(context.IOStream, "-e", query)
+	code, err := context.Environment.SQL().OpenShell(context.IOStream, "-e", query)
 	if err != nil {
 		return err
 	}