Fix EZP-23604: dont reuse session var when installing a different package

https://jira.ez.no/browse/EZP-23604

Problem: After clicking "Install Package" and going back, packageName is stored in persistent data.
It will always be reused until `eZPackageInstallerData` is cleared.

On the first step, however, selecting a new package should *not* reuse session variable.