fix(exasol): case-insensitive alias matching for LOCAL. prefix

[antonireus]

_add_local_prefix_for_aliases did case-sensitive string matching when rewriting alias references to LOCAL.<alias> in WHERE/GROUP BY/HAVING. Exasol folds unquoted identifiers to uppercase, so an alias idField and a reference idfield are the same identifier but the lookup missed, producing invalid SQL:

  sqlglot.transpile(                                                                                                                                              
      "SELECT 1 AS idField FROM t GROUP BY idfield",            
      read="mysql", write="exasol",   
  )
  # before: ... GROUP BY idfield        → Exasol error
  # after:  ... GROUP BY LOCAL.idField  → ok                                                                                                                      

Fix: use dialect.normalize_identifier() to compute the comparison key. The exp.Select preprocessing moves into a select_sql override so self.dialect is available.

We went with this approach, because now Exasol supports setting a different normalization strategy, so the function would work consistently when using an extended Exasol dialect using a different normalization strategy.

Tests added to test_local_prefix_for_alias. Verified against a live Exasol instance.