[SPARK-27184][CORE] Avoid hardcoded 'spark.jars', 'spark.files', 'spark.submit.pyFiles' and 'spark.submit.deployMode'

## What changes were proposed in this pull request?

For [SPARK-27184](https://issues.apache.org/jira/browse/SPARK-27184)

In the `org.apache.spark.internal.config`, we define the variables of `FILES` and `JARS`, we can use them instead of "spark.jars" and "spark.files".

```scala
private[spark] val JARS = ConfigBuilder("spark.jars")
  .stringConf
  .toSequence
  .createWithDefault(Nil)
```

```scala
private[spark] val FILES = ConfigBuilder("spark.files")
  .stringConf
  .toSequence
  .createWithDefault(Nil)
```

Other :
In the `org.apache.spark.internal.config`, we define the variables of `SUBMIT_PYTHON_FILES ` and `SUBMIT_DEPLOY_MODE `, we can use them instead of "spark.submit.pyFiles" and "spark.submit.deployMode".
```scala
private[spark] val SUBMIT_PYTHON_FILES = ConfigBuilder("spark.submit.pyFiles")
    .stringConf
    .toSequence
    .createWithDefault(Nil)

```
```scala
private[spark] val SUBMIT_DEPLOY_MODE = ConfigBuilder("spark.submit.deployMode")
    .stringConf
    .createWithDefault("client")
```

Closes apache#24123 from hehuiyuan/hehuiyuan-patch-6.

Authored-by: hehuiyuan <[email protected]>
Signed-off-by: Hyukjin Kwon <[email protected]>

Author: hehuiyuan

core/src/main/scala/org/apache/spark/SparkContext.scala

@@ -394,7 +394,7 @@ class SparkContext(config: SparkConf) extends Logging {
     _conf.set(EXECUTOR_ID, SparkContext.DRIVER_IDENTIFIER)
 
     _jars = Utils.getUserJars(_conf)
-    _files = _conf.getOption("spark.files").map(_.split(",")).map(_.filter(_.nonEmpty))
+    _files = _conf.getOption(FILES.key).map(_.split(",")).map(_.filter(_.nonEmpty))
       .toSeq.flatten
 
     _eventLogDir =

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

@@ -542,10 +542,10 @@ private[spark] class SparkSubmit extends Logging {
       OptionAssigner(args.totalExecutorCores, STANDALONE | MESOS | KUBERNETES, ALL_DEPLOY_MODES,
         confKey = CORES_MAX.key),
       OptionAssigner(args.files, LOCAL | STANDALONE | MESOS | KUBERNETES, ALL_DEPLOY_MODES,
-        confKey = "spark.files"),
-      OptionAssigner(args.jars, LOCAL, CLIENT, confKey = "spark.jars"),
+        confKey = FILES.key),
+      OptionAssigner(args.jars, LOCAL, CLIENT, confKey = JARS.key),
       OptionAssigner(args.jars, STANDALONE | MESOS | KUBERNETES, ALL_DEPLOY_MODES,
-        confKey = "spark.jars"),
+        confKey = JARS.key),
       OptionAssigner(args.driverMemory, STANDALONE | MESOS | YARN | KUBERNETES, CLUSTER,
         confKey = DRIVER_MEMORY.key),
       OptionAssigner(args.driverCores, STANDALONE | MESOS | YARN | KUBERNETES, CLUSTER,

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

@@ -183,9 +183,9 @@ private[deploy] class SparkSubmitArguments(args: Seq[String], env: Map[String, S
       .orElse(sparkProperties.get(config.CORES_MAX.key))
       .orNull
     name = Option(name).orElse(sparkProperties.get("spark.app.name")).orNull
-    jars = Option(jars).orElse(sparkProperties.get("spark.jars")).orNull
-    files = Option(files).orElse(sparkProperties.get("spark.files")).orNull
-    pyFiles = Option(pyFiles).orElse(sparkProperties.get("spark.submit.pyFiles")).orNull
+    jars = Option(jars).orElse(sparkProperties.get(config.JARS.key)).orNull
+    files = Option(files).orElse(sparkProperties.get(config.FILES.key)).orNull
+    pyFiles = Option(pyFiles).orElse(sparkProperties.get(config.SUBMIT_PYTHON_FILES.key)).orNull
     ivyRepoPath = sparkProperties.get("spark.jars.ivy").orNull
     ivySettingsPath = sparkProperties.get("spark.jars.ivySettings")
     packages = Option(packages).orElse(sparkProperties.get("spark.jars.packages")).orNull
@@ -194,7 +194,7 @@ private[deploy] class SparkSubmitArguments(args: Seq[String], env: Map[String, S
     repositories = Option(repositories)
       .orElse(sparkProperties.get("spark.jars.repositories")).orNull
     deployMode = Option(deployMode)
-      .orElse(sparkProperties.get("spark.submit.deployMode"))
+      .orElse(sparkProperties.get(config.SUBMIT_DEPLOY_MODE.key))
       .orElse(env.get("DEPLOY_MODE"))
       .orNull
     numExecutors = Option(numExecutors)

core/src/test/scala/org/apache/spark/deploy/SparkSubmitSuite.scala

@@ -681,7 +681,7 @@ class SparkSubmitSuite
       appArgs.jars should be(Utils.resolveURIs(jars))
       appArgs.files should be(Utils.resolveURIs(files))
       conf.get(JARS) should be(Utils.resolveURIs(jars + ",thejar.jar").split(",").toSeq)
-      conf.get("spark.files") should be(Utils.resolveURIs(files))
+      conf.get(FILES) should be(Utils.resolveURIs(files).split(",").toSeq)
 
       // Test files and archives (Yarn)
       val clArgs2 = Seq(