[2.1] Fix SessionHandlerInterface autoloading
[2.1] Fix SessionHandlerInterface autoloading

The path for 2.1 is also incorrect. For master, this was fixed in 3b4708. This patch adds the `target-dir` prefix to the autoloading base directory of the HttpFoundation stubs.
ygerasimov/HttpFoundation,symfony/HttpFoundation,mneuhaus/HttpFoundation,oswaldderiemaecker/HttpFoundation,nichdu/HttpFoundation,symfony/http-foundation