Run scala:compile goal when just compile is requested.

Previously, you would need to request explicitly `scala:compile` in
order to compile Scala files. The `scala:compile` would not be run as part
of standard commands like `package` or `install`.

Now scala goals are properly integrated into standard Maven goals.

Author: gkossakowski

maven-sample/pom.xml

@@ -75,6 +75,14 @@
         <groupId>net.alchim31.maven</groupId>
         <artifactId>scala-maven-plugin</artifactId>
         <version>3.1.6</version>
+        <executions>
+          <execution>
+            <goals>
+              <goal>compile</goal>
+              <goal>testCompile</goal>
+            </goals>
+          </execution>
+        </executions>
         <configuration>
           <args>
             <!-- work-around for https://issues.scala-lang.org/browse/SI-8358 -->