[SPARK-26011][SPARK-SUBMIT] Yarn mode pyspark app without python main resource does not honor "spark.jars.packages"

SparkSubmit determines pyspark app by the suffix of primary resource but Livy
uses "spark-internal" as the primary resource when calling spark-submit,
therefore args.isPython is set to false in SparkSubmit.scala.

In Yarn mode, SparkSubmit module is responsible for resolving maven coordinates
and adding them to "spark.submit.pyFiles" so that python's system path can be set correctly.

The fix is to resolve maven coordinates not only when args.isPython is true,
but also when primary resource is spark-internal.

Tested the patch with Livy submitting pyspark app, spark-submit, pyspark with or without packages config.

Signed-off-by: Shanyu Zhao <shzhaomicrosoft.com>

Closes apache#23009 from shanyu/shanyu-26011.

Authored-by: Shanyu Zhao <shzhao@microsoft.com>
Signed-off-by: Sean Owen <sean.owen@databricks.com>
(cherry picked from commit 9a5fda6)
Signed-off-by: Sean Owen <sean.owen@databricks.com>

Author: shanyu

core/src/main/scala/org/apache/spark/deploy/SparkSubmit.scala

@@ -318,7 +318,7 @@ private[spark] class SparkSubmit extends Logging {
 
       if (!StringUtils.isBlank(resolvedMavenCoordinates)) {
         args.jars = mergeFileLists(args.jars, resolvedMavenCoordinates)
-        if (args.isPython) {
+        if (args.isPython || isInternal(args.primaryResource)) {
           args.pyFiles = mergeFileLists(args.pyFiles, resolvedMavenCoordinates)
         }
       }