UI should allow XAR export even when Package plugin is not found

in export_modal.vm we currently have:

#if($hasAdmin && "$!xwiki.getPlugin('package')" != '')

The filter stream application is used by default and thus the Package plugin is not required. It's required only if the xwiki.action.export.xar.usefilter property in xwiki.cfg is set.

The reason for this use case is to be able to work on wiki pages and export them when writing functional tests. This is why we bundle "org.xwiki.platform:xwiki-platform-filter-instance-oldcore" in the PackageMojo: https://github.com/xwiki/xwiki-platform/blob/f70c18544967eb7c92759965d694ff12701c5938/xwiki-platform-tools/xwiki-platform-tool-packager-plugin/src/main/java/org/xwiki/tool/packager/PackageMojo.java#L716-L716