Move eclipse hackery into a profile.

This gets rid of a ton of noise when running mvn with plugin:goal syntax, e.g. source:jar.

pom.xml

@@ -116,7 +116,17 @@
         <version>2.5</version>
       </plugin>
     </plugins>
+  </build>
 
+  <profiles>
+    <profile>
+      <id>eclipse</id>
+      <activation>
+        <property>
+          <name>m2e.version</name>
+        </property>
+      </activation>
+      <build>
         <pluginManagement>
           <plugins>
             <plugin>
@@ -148,8 +158,8 @@
           </plugins>
         </pluginManagement>
       </build>
+    </profile>
 
-  <profiles>
     <profile>
       <id>release-sign-artifacts</id>
       <activation>