Don't expose methods and attrs for internal usage

[kamipo]

No description provided.

[kaspth]

All these classes are nodoc'ed, why not keep the Git history intact and then move to private on Ruby 2.3+?

[kamipo]

I changed to keep the Git history intact as much as possible.

I think that correct visibility helps to understand the code (a reason like #26076).

The TODO comments in the previous implementation followed 21e5fd4. Probably the whole of the TODO comments will be removed and will be changed attrs visibility to private in the future.

[matthewd]

This isn't really private, so the change seems more misleading than explanatory.

[kaspth]

This seems worse than the originally proposed protected to me. History be damned.

[kamipo]

Okay I'll change it back to originally proposed implementation.

[kaspth]

Regardless of what I've said in the past, I think we should just leave these be and deal with them once we're Ruby 2.3+.

@matthewd your call 👍

[kamipo]

This isn't really private, so the change seems more misleading than explanatory.

Good point. Actually actual_source_reflection is called in ThroughReflection.
But ThroughReflection don't inherite AssociationReflection, so I moved actual_source_reflection from AssociationReflection to AbstractReflection.

[kamipo]

Actually actual_source_reflection is called in ThroughReflection.
But ThroughReflection don't inherite AssociationReflection, so I moved actual_source_reflection from AssociationReflection to AbstractReflection.