Fixing locale directory for Packages installed with Composer

I've installed `Horde_Imap_Client` package with Composer, but it fails to load the translation files.

Those packages results in having the data/locale directory at `/path/to/pear-pear.horde.org/Horde_Package/data/locale` while translations are trying to access `/path/to/pear-pear.horde.org/Horde_Package/data/Horde_Package/locale`

On a related note: it would be great if `@data_dir@` is not "hardcoded" as in not having the absolute path on it but rather use PHP functions like [dirname](php.net/dirname) to define it.

It is necessary when you have a build server, for example. At my daily job releases are prepared by a build server, which effectively does a `composer install`, but it does not quite work because the paths are different from the target server. :/