In Active Record internal, `arel_table` is not directly used but
`arel_attribute` is used, since `arel_table` doesn't normalize an
attribute name as a string, and doesn't resolve attribute aliases.

For the above reason, `arel_attribute` should be used rather than
`arel_table`, but most people directly use `arel_table`, both
`arel_table` and `arel_attribute` are private API though.

Although I'd not recommend using private API, `arel_table` is actually
widely used, and it is also problematic for unscopeable queries and
hash-like relation merging friendly, as I explained at #39863.

To resolve the issue, this change moves Arel attribute normalization
(attribute name as a string, and attribute alias resolution) into
`arel_table`.