Reorganizing pom.xml

- consolidate versions
- enable error-prone
- fix issue detected by error-prone

Change-Id: Ie2f5d53b7357ec43dd00e56097f7163ca1a63094
diff --git a/compiler/plugin/maven/pom.xml b/compiler/plugin/maven/pom.xml
index b91aaa8..4ebca78 100644
--- a/compiler/plugin/maven/pom.xml
+++ b/compiler/plugin/maven/pom.xml
@@ -56,10 +56,25 @@
             <artifactId>maven-artifact</artifactId>
             <version>3.3.9</version>
         </dependency>
+
         <dependency>
+            <groupId>com.google.guava</groupId>
+            <artifactId>guava</artifactId>
+        </dependency>
+
+        <dependency>
+            <!-- FIXME avoid using alpha version -->
             <groupId>org.apache.maven</groupId>
             <artifactId>maven-project</artifactId>
             <version>3.0-alpha-2</version>
+            <exclusions>
+                <exclusion>
+                    <!-- exclude from transitive dependency -->
+                    <!-- since this has package collision with recent guava -->
+                    <artifactId>com.google.code.google-collections</artifactId>
+                    <groupId>google-collection</groupId>
+                </exclusion>
+            </exclusions>
         </dependency>
         <dependency>
             <groupId>org.apache.maven</groupId>
@@ -75,7 +90,6 @@
         <dependency>
             <groupId>org.slf4j</groupId>
             <artifactId>slf4j-api</artifactId>
-            <version>1.7.21</version>
             <scope>provided</scope>
         </dependency>
         <dependency>