[SPARK-49370][BUILD] maven.scaladoc.skip should not affect test code compilation

[pan3793]

What changes were proposed in this pull request?

Move maven.scaladoc.skip from the scala-maven-plugin's global configuration to its attach-scaladocs execution's configuration.

Why are the changes needed?

maven.scaladoc.skip was added in SPARK-43461 to speed up the dev/make-distribution.sh by skipping scaladocs generation, but it was wrongly applied to a broader scope.

Does this PR introduce any user-facing change?

It affects developers who use build/mvn deploy -Dmaven.scaladoc.skip=true to deploy the artifacts case, without this patch, wrong <module>-tests.jars (don't contain the test classes but only resources) will be produced.

How was this patch tested?

build/mvn clean package -DskipTests -Dmaven.scaladoc.skip=true -pl core -am

before

$ ll core/target/spark-core_2.13-4.0.0-SNAPSHOT-tests.jar
-rw-r--r--@ 1 chengpan  staff   381K Aug 23 23:55 core/target/spark-core_2.13-4.0.0-SNAPSHOT-tests.jar

$ jar tf core/target/spark-core_2.13-4.0.0-SNAPSHOT-tests.jar | grep class | head
<nothing>

after

$ ll core/target/spark-core_2.13-4.0.0-SNAPSHOT-tests.jar
-rw-r--r--@ 1 chengpan  staff   5.7M Aug 24 00:12 core/target/spark-core_2.13-4.0.0-SNAPSHOT-tests.jar

$ jar tf core/target/spark-core_2.13-4.0.0-SNAPSHOT-tests.jar | grep class | head
test/org/apache/spark/JavaTaskContextCompileCheck.class
test/org/apache/spark/Java8RDDAPISuite.class
test/org/apache/spark/JavaAPISuite$SomeCustomClass.class
test/org/apache/spark/JavaAPISuite$BuggyMapFunction.class
test/org/apache/spark/JavaAPISuite$DoubleComparator.class
test/org/apache/spark/JavaAPISuite.class
test/org/apache/spark/JavaTaskContextCompileCheck$JavaTaskCompletionListenerImpl.class
test/org/apache/spark/JavaTaskContextCompileCheck$JavaTaskFailureListenerImpl.class
test/org/apache/spark/JavaAPISuite$Class2.class
test/org/apache/spark/JavaAPISuite$1.class

Was this patch authored or co-authored using generative AI tooling?

No.

[pan3793]

cc @wangyum @LuciferYang

[HyukjinKwon]

Merged to master.