Bake version numbers in at build-time

`getPackage().getImplementationVersion()` may not be reliable, see:
352aac7#commitcomment-25048691

MOE_MIGRATED_REVID=172638406

Author: cushon

core/pom.xml

@@ -191,6 +191,48 @@
           </execution>
         </executions>
       </plugin>
+      <plugin>
+        <groupId>com.google.code.maven-replacer-plugin</groupId>
+        <artifactId>replacer</artifactId>
+        <version>1.5.3</version>
+        <executions>
+          <execution>
+            <phase>generate-sources</phase>
+            <goals>
+              <goal>replace</goal>
+            </goals>
+          </execution>
+        </executions>
+        <configuration>
+          <file>core/src/main/java/com/google/googlejavaformat/java/GoogleJavaFormatVersion.java.template</file>
+          <outputFile>${project.build.directory}/generated-sources/java/com/google/googlejavaformat/java/GoogleJavaFormatVersion.java</outputFile>
+          <replacements>
+            <replacement>
+              <token>%VERSION%</token>
+              <value>${project.version}</value>
+            </replacement>
+          </replacements>
+        </configuration>
+      </plugin>
+      <plugin>
+        <groupId>org.codehaus.mojo</groupId>
+        <artifactId>build-helper-maven-plugin</artifactId>
+        <version>3.0.0</version>
+        <executions>
+          <execution>
+            <id>add-source</id>
+            <phase>generate-sources</phase>
+            <goals>
+              <goal>add-source</goal>
+            </goals>
+            <configuration>
+              <sources>
+                <source>${project.build.directory}/generated-sources/java/</source>
+              </sources>
+            </configuration>
+          </execution>
+        </executions>
+      </plugin>
     </plugins>
   </build>
 </project>

core/src/main/java/com/google/googlejavaformat/java/GoogleJavaFormatVersion.java renamed to core/src/main/java/com/google/googlejavaformat/java/GoogleJavaFormatVersion.java.template

@@ -18,8 +18,7 @@
 
 class GoogleJavaFormatVersion {
 
-  static Object version() {
-    return Optional.ofNullable(Main.class.getPackage().getImplementationVersion())
-        .orElse("unknown");
+  static String version() {
+    return "%VERSION%";
   }
 }