[FIXED JENKINS-13754]

while I can't pinpoint what's really causing this, moving this
dependency to core seems to solve the problem.

Guava is defined as dependency in org.kohsuke.stapler:stapler, then
redefined to a newer version in maven-plugin. The latter gets to plugins
as a test scope. When I look at the dependency scope of this in a plugin
project in IntelliJ IDEA, it reports this dependency as the test scope
(and this makes the error somewhat understandable.)

core/pom.xml

@@ -600,6 +600,11 @@ THE SOFTWARE.
       <scope>system</scope>
       <systemPath>/usr/local/yjp/lib/yjp.jar</systemPath>
     </dependency-->
+
+      <dependency>
+        <groupId>com.google.guava</groupId>
+        <artifactId>guava</artifactId>
+      </dependency>
   </dependencies>
 
   <build>

maven-plugin/pom.xml

@@ -202,11 +202,6 @@ THE SOFTWARE.
       </exclusions>
     </dependency>
 
-    <dependency>
-      <groupId>com.google.guava</groupId>
-      <artifactId>guava</artifactId>
-    </dependency>
-
     <dependency>
       <groupId>org.sonatype.sisu</groupId>
       <artifactId>sisu-inject-bean</artifactId>