ArchLinux packages need working symlinks in $srcdir

fixes https://github.com/openSUSE/open-build-service/issues/658

problem:

  `makepkg -o` does (with `PWD=$TOPDIR/SOURCES`):

    mkdir src
    for s in ${source[@]}; do
      ln -s $SRCDEST/$s src/
    done

  where `$SRCDEST` is `$PWD` unless overridden.

  then we move `$TOPDIR/SOURCES/*` (`s/SOURCES/BUILD/`), and the
  symlinks still contain the old paths.

solution:

  run `makepkg -o` in `$TOPDIR/BUILD`.