[SPARK-25298][BUILD] Improve build definition for Scala 2.12

## What changes were proposed in this pull request?

Improve build for Scala 2.12. Current build for sbt fails on the subproject `repl`:

```
[info] Compiling 6 Scala sources to /Users/rendong/wdi/spark/repl/target/scala-2.12/classes...
[error] /Users/rendong/wdi/spark/repl/scala-2.11/src/main/scala/org/apache/spark/repl/SparkILoopInterpreter.scala:80: overriding lazy value importableSymbolsWithRenames in class ImportHandler of type List[(this.intp.global.Symbol, this.intp.global.Name)];
[error]  lazy value importableSymbolsWithRenames needs `override' modifier
[error]       lazy val importableSymbolsWithRenames: List[(Symbol, Name)] = {
[error]                ^
[warn] /Users/rendong/wdi/spark/repl/src/main/scala/org/apache/spark/repl/SparkILoop.scala:53: variable addedClasspath in class ILoop is deprecated (since 2.11.0): use reset, replay or require to update class path
[warn]       if (addedClasspath != "") {
[warn]           ^
[warn] /Users/rendong/wdi/spark/repl/src/main/scala/org/apache/spark/repl/SparkILoop.scala:54: variable addedClasspath in class ILoop is deprecated (since 2.11.0): use reset, replay or require to update class path
[warn]         settings.classpath append addedClasspath
[warn]                                   ^
[warn] two warnings found
[error] one error found
[error] (repl/compile:compileIncremental) Compilation failed
[error] Total time: 93 s, completed 2018-9-3 10:07:26
```

## How was this patch tested?

```
./dev/change-scala-version.sh 2.12

##  For Maven
./build/mvn -Pscala-2.12 [mvn commands]
##  For SBT
sbt -Dscala.version=2.12.6
```

Closes apache#22310 from sadhen/SPARK-25298.

Authored-by: Darcy Shen <[email protected]>
Signed-off-by: Sean Owen <[email protected]>

Author: da-liii

docs/building-spark.md

@@ -256,3 +256,19 @@ On Linux, this can be done by `sudo service docker start`.
 or
 
     ./build/sbt docker-integration-tests/test
+
+## Change Scala Version
+
+To build Spark using another supported Scala version, please change the major Scala version using (e.g. 2.12):
+
+    ./dev/change-scala-version.sh 2.12
+
+For Maven, please enable the profile (e.g. 2.12):
+
+    ./build/mvn -Pscala-2.12 compile
+
+For SBT, specify a complete scala version using (e.g. 2.12.6):
+
+    ./build/sbt -Dscala.version=2.12.6
+
+Otherwise, the sbt-pom-reader plugin will use the `scala.version` specified in the spark-parent pom.

project/SparkBuild.scala

@@ -94,6 +94,12 @@ object SparkBuild extends PomBuild {
       case Some(v) =>
         v.split("(\\s+|,)").filterNot(_.isEmpty).map(_.trim.replaceAll("-P", "")).toSeq
     }
+
+    Option(System.getProperty("scala.version"))
+      .filter(_.startsWith("2.12"))
+      .foreach { versionString =>
+        System.setProperty("scala-2.12", "true")
+      }
     if (System.getProperty("scala-2.12") == "") {
       // To activate scala-2.10 profile, replace empty property value to non-empty value
       // in the same way as Maven which handles -Dname as -Dname=true before executes build process.

repl/pom.xml

@@ -32,8 +32,8 @@
 
   <properties>
     <sbt.project.name>repl</sbt.project.name>
-    <extra.source.dir>scala-2.11/src/main/scala</extra.source.dir>
-    <extra.testsource.dir>scala-2.11/src/test/scala</extra.testsource.dir>
+    <extra.source.dir>src/main/scala-${scala.binary.version}</extra.source.dir>
+    <extra.testsource.dir>src/test/scala-${scala.binary.version}</extra.testsource.dir>
   </properties>
 
   <dependencies>
@@ -167,14 +167,4 @@
     </plugins>
   </build>
 
-  <profiles>
-    <profile>
-      <id>scala-2.12</id>
-      <properties>
-        <extra.source.dir>scala-2.12/src/main/scala</extra.source.dir>
-        <extra.testsource.dir>scala-2.12/src/test/scala</extra.testsource.dir>
-      </properties>
-    </profile>
-  </profiles>
-
 </project>