[maven] properly attach the CEP Scala source code

Two options, either change the default Maven source directory from
'src/main/java' to 'src/main/scala' or use the build-helper-maven-plugin
to attach the Scala sources. Opting for both here to be in lines with
Maven standards and support Eclipse.

This closes apache#2908.

flink-libraries/flink-cep-scala/pom.xml

@@ -93,6 +93,8 @@ under the License.
  </dependencies>
 
  <build>
+ <!-- Not strictly necessary because of the build-helper-maven-plugin below. -->
+ <sourceDirectory>src/main/scala</sourceDirectory>
  <plugins>
  <!-- Scala Compiler -->
  <plugin>
@@ -116,6 +118,41 @@ under the License.
  <configLocation>${project.basedir}/../../tools/maven/scalastyle-config.xml</configLocation>
  </configuration>
  </plugin>
+
+ <!-- Adding scala source directories to build path for Eclipse and generating the sources jar -->
+ <plugin>
+ <groupId>org.codehaus.mojo</groupId>
+ <artifactId>build-helper-maven-plugin</artifactId>
+ <version>1.7</version>
+ <executions>
+ <!-- Add src/main/scala to eclipse build path -->
+ <execution>
+ <id>add-source</id>
+ <phase>generate-sources</phase>
+ <goals>
+ <goal>add-source</goal>
+ </goals>
+ <configuration>
+ <sources>
+ <source>src/main/scala</source>
+ </sources>
+ </configuration>
+ </execution>
+ <!-- Add src/test/scala to eclipse build path -->
+ <execution>
+ <id>add-test-source</id>
+ <phase>generate-test-sources</phase>
+ <goals>
+ <goal>add-test-source</goal>
+ </goals>
+ <configuration>
+ <sources>
+ <source>src/test/scala</source>
+ </sources>
+ </configuration>
+ </execution>
+ </executions>
+ </plugin>
  </plugins>
  </build>