track item schema names to identify name collisions w/ default schema

track item schema names to identify name collisions w/ default schema

Added an additional lookup step to the compiler which will track all FROM
clauses which are tables, that may have the same name shared in multiple
schemas where one of the schemas is the implicit "default" schema; in this
case, the table name when referring to that name without a schema
qualification will be rendered with an anonymous alias name at the compiler
level in order to disambiguate the two (or more) names. The approach of
schema-qualifying the normally unqualified name with the server-detected
"default schema name" value was also considered, however this approach
doesn't apply to Oracle nor is it accepted by SQL Server, nor would it work
with multiple entries in the PostgreSQL search path. The name collision
issue resolved here has been identified as affecting at least Oracle,
PostgreSQL, SQL Server, MySQL and MariaDB.

Fixes: #7471
Change-Id: Id65e7ca8c43fe8d95777084e8d5ec140ebcd784d