CRM_Utils_SQL_Select - The default alias for subquery should be more distinctive

CRM_Utils_SQL_Select - The default alias for subquery should be more distinctive

1. If you're writing basic SQL from scratch, you would expect the symbol `a` to be available to you
   (e.g. for use in column-aliases or join-aliases). It shouldn't be a reserved word.

2. In the case of Api4EntitySetQuery, it's important to use the alias as `a` - because that matches the
   rest of the api4 query-building.