Fix lookup of source files in non-Poedit POs
Fix lookup of source files in non-Poedit POs

Fix breakage caused by the changes in
9bdeafd98baf5e533d58e630d95a003189ecc843: it broke showing source code
files if the PO file wasn't configured for extraction in Poedit (i.e.
didn't have paths headers) _and_ had source locations relative to some
path other than the PO's location.a

Fixes #259.

nawawi/poedit,c72578/poedit,alexhenrie/poedit,nawawi/poedit,vslavik/poedit,c72578/poedit,c72578/poedit,nawawi/poedit,nawawi/poedit,c72578/poedit,c72578/poedit,vslavik/poedit,PKRoma/poedit,alexhenrie/poedit,PKRoma/poedit,nawawi/poedit,PKRoma/poedit,vslavik/poedit,PKRoma/poedit,alexhenrie/poedit,vslavik/poedit,alexhenrie/poedit,alexhenrie/poedit,vslavik/poedit,PKRoma/poedit