kde_projects: Follow symlinks to ensure metadata repo is read.

Into the migration to Gitlab, kdesrc-build was oddly failing to
understand the KDE project database.

Long story short, the sysadmins tried to help the various scripts by
placing a symlink from the old module location to the new
'invent-modules' in repo-metadata. However kdesrc-build's search logic
didn't follow symlinks since Perl's File::Find doesn't do this by
default.

It's an easy option to add now though, so I've done that and tried to
add some better logic checks / error messages for similar problems that
might recur in the future.

kdesrc-build still fails to build many modules though because the
metadata key we're checking doesn't give the right path? I'll look into
that more...