[SPARK-14465][BUILD] Checkstyle should check all Java files

## What changes were proposed in this pull request?

Currently, `checkstyle` is configured to check the files under `src/main/java`. However, Spark has Java files in `src/main/scala`, too. This PR fixes the following configuration in `pom.xml` and the unchecked-so-far violations on those files.
```xml
-<sourceDirectory>${basedir}/src/main/java</sourceDirectory>
+<sourceDirectories>${basedir}/src/main/java,${basedir}/src/main/scala</sourceDirectories>
```

## How was this patch tested?

After passing the Jenkins build and manually `dev/lint-java`. (Note that Jenkins does not run `lint-java`)

Author: Dongjoon Hyun <dongjoon@apache.org>

Closes #12242 from dongjoon-hyun/SPARK-14465.

1 files changed, 1 insertions, 1 deletions

diff --git a/pom.xml b/pom.xml
index 3f9e4abc32..4cbc6a2f11 100644
--- a/pom.xml
+++ b/pom.xml
@@ -2253,7 +2253,7 @@
           <failOnViolation>false</failOnViolation>
           <includeTestSourceDirectory>true</includeTestSourceDirectory>
           <failOnWarning>false</failOnWarning>
-          <sourceDirectory>${basedir}/src/main/java</sourceDirectory>
+          <sourceDirectories>${basedir}/src/main/java,${basedir}/src/main/scala</sourceDirectories>
           <testSourceDirectory>${basedir}/src/test/java</testSourceDirectory>
           <configLocation>dev/checkstyle.xml</configLocation>
           <outputFile>${basedir}/target/checkstyle-output.xml</outputFile>