fix file path for Arch repo

[YukariChiba]

Currently OBS has a problem with file lookups for the Arch repo, resulting in potentially incorrectly generated .files files (missing path prefixes).

As described in https://perldoc.perl.org/Archive::Tar::File :

$path = $file->full_path
Returns the full path from the tar header; this is basically a concatenation of the prefix and name fields.

Using only name will result in the loss of the prefix part, e.g. usr/lib or usr/lib/python3.xx in some python packages.