MongoDataAutoConfiguration now sets up packages for @Document scanning.

MongoDataAutoConfiguration now sets up the domain type scanning for packages
defined via spring-data-mongodb.mappingBasePackages and falls back to all
auto-configuration packages if nothing was configured explicitly.

This currently implemented by effectively copying AbstractMongoConfiguration's
getInitialEntitySet() but we can remove that as soon as we've exposed that
functionality in Spring Data MongoDB's config class.

Issue: #2107.