Previously, if a BeanPostProcessor bean was declared in a configuration
class that depended on the persistence context, packages to scan would
not be configured on the LocalContainerEntityManagerFactoryBean. This
was due to the BeanPostProcessor bean triggering the early instantiation
of the LCEMFB before EntityScanBeanPostProcessor, the BeanPostProcessor,
that applies the @EntityScan configuration, had been given a chance to
configure it.

This commit updates EntityScanBeanPostProcessor to implement Ordered
with an order of zero. This ensures that its ordering is predictable
and that it will be driven before any unordered BeanPostProcessor. This
means that, unless the user has specifically ordered their
BeanPostProcessor to run before EntityScanBeanPostProcessor, 
LCEMFB’s packages to scan will be configured as expected.

Fixes gh-2478