PackagePersistence#getPackages should ignore files

Because the packages are stored in folder, the method PackagePersistence#getPackages should ignore files when iterating on each item of the packages store.

This will avoid errors like

Could not read package list
org.nuxeo.connect.update.PackageException: Invalid package - no package.xml file found in package README.txt
	at org.nuxeo.connect.update.standalone.LocalPackageImpl.<init>(LocalPackageImpl.java:107)

if somebody adds a plain file under $DATA/packages/store