Ignore/exclude IDEA-generated classpath index files (#2094)

See
https://youtrack.jetbrains.com/issue/IDEA-305759/Gradle-cannot-handle-classpath.index-duplicates
for more info -- it looks like this is fixed in some recent versions but
as of right now it doesn't seem like it's fixed on the Google-managed
version

.gitignore

@@ -79,6 +79,7 @@ autogenerated/
 **/*.iml
 nomulus.ipr
 nomulus.iws
+**/classpath.index
 
 # Auto-generated java classes by Intellij
 */src/main/generated/

java_common.gradle

@@ -193,3 +193,10 @@ jacocoTestCoverageVerification {
     }
   }
 }
+
+project.tasks.withType(Copy) {
+  duplicatesStrategy = DuplicatesStrategy.EXCLUDE
+}
+project.tasks.withType(Jar) {
+  duplicatesStrategy = DuplicatesStrategy.EXCLUDE
+}