Use DefaultPersistenceUnitManager instead of Merging-~

We don't need the MergingPersistenceUnitManager's features so limit the diff
from 1-initial as much as possible.
Use more specific package scan path
Get rid of persistence.xml

Use LocalContainerEntityManagerFactoryBean's packagesToScan-property to specify
the base classpath and no longer define an explicit PersistenceUnitManager.
This allows us to completely remove the persistence.xml file in this scenario.
Remove explicit creation of a persistence unit manager

As long as we don't configure the persistence unit manager in unusual ways, we
can simply use the LocalContainerEntityManagerFactoryBean's default persistence
unit manager.
Format code
Merge branch '1-initial' into 2-jpa-classpathscan