Move SelectQueryable to Dialect module

[hfhbd]

Use-case:
Implement QueryWithResults defined in dialect module in the DB2 dialect but I also need to create SelectQueryable

override fun queryWithResults(sqlStmt: SqlStmt): QueryWithResults? {
  val ext = sqlStmt.extensionStmt
  if (ext != null) {
    val set = (ext as Db2ExtensionStmt).setStmt
    set.compoundSelectStmtInternal?.let { SelectQueryable(it) }
    return Db2SetQueryWithResults(set)
  }
  val compoundSelectStmt = sqlStmt.compoundSelectStmt
  return if (compoundSelectStmt is Db2CompoundSelectMixin) {
    return compoundSelectStmt.queryWithResults()
  } else parentResolver.queryWithResults(sqlStmt)
}