Create a source target rather than embedding sources

This way server deployments can include only the binary bytecode,
but clients can add the source JAR to their maven dependency list:

    <dependency>
      <groupId>gwtexpui</groupId>
      <artifactId>gwtexpui</artifactId>
      <version>${gwtexpuiVersion}</version>
      <scope>compile</scope>
      <type>jar</type>
    </dependency>
    <dependency>
      <groupId>gwtexpui</groupId>
      <artifactId>gwtexpui</artifactId>
      <version>${gwtexpuiVersion}</version>
      <classifier>sources</classifier>
      <scope>provided</scope>
      <type>jar</type>
    </dependency>

Signed-off-by: Shawn O. Pearce <sop@google.com>
diff --git a/pom.xml b/pom.xml
index 351fa80..b0a79c1 100644
--- a/pom.xml
+++ b/pom.xml
@@ -257,12 +257,6 @@
   </properties>
 
   <build>
-    <resources>
-      <resource>
-        <directory>src/main/java</directory>
-      </resource>
-    </resources>
-
     <plugins>
       <plugin>
         <groupId>org.apache.maven.plugins</groupId>
@@ -273,6 +267,16 @@
           <encoding>UTF-8</encoding>
         </configuration>
       </plugin>
+      <plugin>
+        <artifactId>maven-source-plugin</artifactId>
+        <executions>
+          <execution>
+            <goals>
+              <goal>jar</goal>
+            </goals>
+          </execution>
+        </executions>
+      </plugin>
     </plugins>
   </build>