DS-2071 Fix DSpace Import to accept zip, adding subfolder to sourceDir

Addresses an issue with Item Import, using Zip content.
https://jira.duraspace.org/browse/DS-2071

The JIRA Ticket DS-2071 explains the issue, where the subdirectory that the zipped content was being extracted to, wasn't being specified as the sourceDir, thus, when it became time for the importer to pull in the extracted content of the ZIP, it had the parent of the sourceDir, which wasn't a valid SimpleArchiveFormat. This detects when the entries of the zip are in the sub-subdirectories, and sets that common subdirectory as the sourceDir for the zip importer to pass along to the regular DSpace Item Importer.