the symfony's classloader works great:)
In this case, it's a php syntax problem.
You need to import the namespace with "use" or more easily you just need a backslash in front of the classname
for example:
namespace App\Package1;
use App\Interface1;
Class1 implements Interface1
{
//implement some functions here...
}
or
namespace App\Package1;
Class1 implements \App\Interface1;
{
//implement some functions here...
}