ReferencedBy constructor
A table whose foreign keys reference the table being altered, with its indexes.
Carried on AlterTable so a dialect that must REBUILD the altered table
(SQLite) can rebuild its dependents too, dropping a table that is still
referenced fires ON DELETE CASCADE on the referencing rows, silently.
Dialects that alter in place (Postgres) ignore this.
Implementation
const ReferencedBy({required this.table, this.indexes = const []});