[AIRFLOW-5210] Make finding template files more efficient (#5815)
[AIRFLOW-5210] Make finding template files more efficient (#5815)

For large DAGs, iterating over template fields to find template files can be time intensive.
Save this time for tasks that do not specify a template file extension.
wileeam/airflow,nathanielvarona/airflow,wooga/airflow,bolkedebruin/airflow,mtagle/airflow,apache/incubator-airflow,mtagle/airflow,Fokko/incubator-airflow,airbnb/airflow,mrkm4ntr/incubator-airflow,cfei18/incubator-airflow,sekikn/incubator-airflow,spektom/incubator-airflow,apache/airflow,Acehaidrey/incubator-airflow,nathanielvarona/airflow,mistercrunch/airflow,lyft/incubator-airflow,apache/airflow,spektom/incubator-airflow,nathanielvarona/airflow,Fokko/incubator-airflow,dhuang/incubator-airflow,danielvdende/incubator-airflow,dhuang/incubator-airflow,nathanielvarona/airflow,wooga/airflow,wileeam/airflow,mrkm4ntr/incubator-airflow,sekikn/incubator-airflow,cfei18/incubator-airflow,danielvdende/incubator-airflow,Fokko/incubator-airflow,wileeam/airflow,bolkedebruin/airflow,DinoCow/airflow,sekikn/incubator-airflow,apache/airflow,mistercrunch/airflow,airbnb/airflow,danielvdende/incubator-airflow,mistercrunch/airflow,cfei18/incubator-airflow,wooga/airflow,dhuang/incubator-airflow,nathanielvarona/airflow,Acehaidrey/incubator-airflow,cfei18/incubator-airflow,wooga/airflow,apache/airflow,bolkedebruin/airflow,danielvdende/incubator-airflow,lyft/incubator-airflow,lyft/incubator-airflow,Acehaidrey/incubator-airflow,bolkedebruin/airflow,wileeam/airflow,danielvdende/incubator-airflow,Acehaidrey/incubator-airflow,apache/airflow,mistercrunch/airflow,DinoCow/airflow,apache/incubator-airflow,mrkm4ntr/incubator-airflow,bolkedebruin/airflow,Acehaidrey/incubator-airflow,airbnb/airflow,apache/incubator-airflow,cfei18/incubator-airflow,apache/airflow,Acehaidrey/incubator-airflow,spektom/incubator-airflow,danielvdende/incubator-airflow,mrkm4ntr/incubator-airflow,Fokko/incubator-airflow,DinoCow/airflow,DinoCow/airflow,dhuang/incubator-airflow,cfei18/incubator-airflow,spektom/incubator-airflow,mtagle/airflow,sekikn/incubator-airflow,mtagle/airflow,lyft/incubator-airflow,airbnb/airflow,nathanielvarona/airflow,apache/incubator-airflow