At present, auto-configuration of `LdapContextSource` is conditional on
presence of a `ContextSource` bean. However, there are valid use cases
which require multiple `ContextSource` bean, for instance
`PooledContextSource`. With the current arrangement, the
auto-configuration of `LdapContextSource` will back off if user provides
a `PooledContextSource` bean, while it would still be reasonable to
reuse the auto-configured `LdapContextSource`.

This commit improves `LdapContextSource` factory method return value and
condition to back off only if users actually provide a
`LdapContextSource` bean themselves.

See gh-13143