Fix generating the metadata.json file

kpackage_install_package macro takes dir as argument, some usage of it
passes it directory path instead of directory itself, as in foo/bar
instead of bar. 5ce8fe34

 commit broke this behavior.

This fixes it by replacing name of temporary json file to
$component-metadata.json instead of $dir/metadata.json.

CCMAIL: aleixpol@kde.org
CCMAIL: release-team@kde.org
CCMAIL: faure@kde.org

Reviewed-by: Marco Martin <notmart@gmail.com>

David, can you please re-do 5.28 kpackage tarball with this commit?