Installing extension bringes 'Package "xy" is not available'
If I copy an extension directory to typo3conf/ext/ and try to install it using the Extension Manager (it is listed already), I get:
#1166546734: Package "xy" is not available. Please check if the package exists and that the package key is correct (package keys are case sensitive). (More information)
The problem is that the PackageStates file is not up-to-date: it does not include this new package which was just copied there without the knowledge of the Package Management.
Since this is not so uncommon, I would propose to re-create the PackageStates.php file as soon as the Extension Manager tries to do something with packages to make sure it is in sync with the reality - just like it behaved before.