Build: Add unused imports check for scala code

[jackylee-ch]

As mentioned in #14342, this PR introduces a compiler check for unused imports in Scala source code.
For Scala 2.13, unused imports will now be treated as errors.

[manuzhang]

Is there a way to auto-fix unused imports?

[jackylee-ch]

Is there a way to auto-fix unused imports?

Yes, we can use Scalafix to automatically remove unused imports.
I’ve already applied it to fix Scala code in #14311. We can add the ScalaFix in a separate PR.

[huaxingao]

@jackylee-ch Thanks for the PR! With your fix, are we supposed to get error for unused imports? I quickly tried it out on my local (4.0_2.13), somehow I got warning instead.

[jackylee-ch]

Could you please share your build command? Have you configured the -DscalaVersion=2.13 parameter? Currently, this error typically occurs only when the Scala version is explicitly set to 2.13, as demonstrated in the example below.

./gradlew build -DflinkVersions= -DkafkaVersions= -DsparkVersions=4.0 -DscalaVersion=2.13 -x test -x integrationTest -x javaDoc

Task :iceberg-spark:iceberg-spark-extensions-4.0_2.13:compileScala

[Error] /Code/iceberg/spark/v4.0/spark-extensions/src/main/scala/org/apache/iceberg/spark/extensions/IcebergSparkSessionExtensions.scala:22:25: Unused import
Applicable -Wconf / @nowarn filters for this fatal warning: msg=, cat=unused-imports, site=org.apache.iceberg.spark.extensions
one error found

[huaxingao]

@jackylee-ch Thanks for the clarification. LGTM

[jackylee-ch]

@manuzhang @huaxingao Could we merge this PR now?
I’m planning to add ScalaFix to automatically remove unused imports, and it depends on the changes in current PR.

[huaxingao]

Thanks @jackylee-ch for the PR!

[jackylee-ch]

Thanks for your reviewing. @manuzhang @huaxingao