Bring back private class methods accessibility in named scope

The receiver in a scope was changed from `klass` to `relation` itself
for all scopes (named scope, default_scope, and association scope)
behaves consistently.

In addition. Before 5.2, if both an AR model class and a Relation
instance have same named methods (e.g. `arel_attribute`,
`predicate_builder`, etc), named scope doesn't respect relation instance
information.

For example:

```ruby
class Post < ActiveRecord::Base
  has_many :comments1, class_name: "RecentComment1"
  has_many :comments2, class_name: "RecentComment2"
end

class RecentComment1 < ActiveRecord::Base
  self.table_name = "comments"
  default_scope { where(arel_attribute(:created_at).gteq(2.weeks.ago)) }
end

class RecentComment2 < ActiveRecord::Base
  self.table_name = "comments"
  default_scope { recent_updated }
  scope :recent_updated, -> { where(arel_attribute(:updated_at).gteq(2.weeks.ago)) }
end
```

If eager loading `Post.eager_load(:comments1, :comments2).to_a`,
`:comments1` (default_scope) respects aliased table name, but
`:comments2` (using named scope) may not work correctly since named
scope doesn't respect relation instance information. See also 801ccab.

But this is a breaking change between releases without deprecation.
I decided to bring back private class methods accessibility in named
scope.

Fixes #31740.
Fixes #32331.

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