Avoid parsing the same import multiple times
Avoid parsing the same import multiple times

In some scenarios the hierarchy of schemas may be very interconnected
and redundant parsing of the same imported files is a huge performance
overhead for code generation (this change showed 20x improvement when
compiling a sample set of ~900 interconnected schemas).

innovimax/bond,upsoft/bond,alfpark/bond,sapek/bond,tstein/bond,sapek/bond,chwarr/bond,jdubrule/bond,sapek/bond,sapek/bond,Microsoft/bond,chwarr/bond,ant0nsc/bond,jdubrule/bond,Microsoft/bond,chwarr/bond,ant0nsc/bond,alfpark/bond,tstein/bond,Microsoft/bond,gencer/bond,tstein/bond,tstein/bond,ant0nsc/bond,ant0nsc/bond,chwarr/bond,Microsoft/bond,upsoft/bond,innovimax/bond,gencer/bond,gencer/bond,jdubrule/bond,tstein/bond,alfpark/bond,Microsoft/bond,chwarr/bond,chwarr/bond,jdubrule/bond,gencer/bond,jdubrule/bond,Microsoft/bond,alfpark/bond,upsoft/bond,innovimax/bond,tstein/bond,jdubrule/bond,sapek/bond,gencer/bond,sapek/bond,innovimax/bond,upsoft/bond