don't specify path to 3rdparty directory when registering Pimple autoloader

The core 3rdparty directory is in the include_path from lib/base.php anyway, so this is unnecessary, and causes problems for downstream distributors who unbundle Pimple.
Merge pull request #6601 from AdamWill/pimple-include

don't specify path to 3rdparty directory when registering Pimple autoloader