build.gradle: Modernize japicmp confiuration

Even though we don't really use japicmp, the configuration was resolving
artifacts even when the task was not executed. After some clean up, the
configuration looks more ordinary.

build.gradle

@@ -417,31 +417,25 @@ subprojects {
         def baselineGrpcVersion = '1.6.1'
 
         // Get the baseline version's jar for this subproject
-        File baselineArtifact = null
-        // Use a detached configuration, otherwise the current version's jar will take precedence
-        // over the baseline jar.
+        configurations {
+            baselineArtifact
+        }
         // A necessary hack, the intuitive thing does NOT work:
         // https://discuss.gradle.org/t/is-the-default-configuration-leaking-into-independent-configurations/2088/6
         def oldGroup = project.group
         try {
             project.group = 'virtual_group_for_japicmp'
-            String depModule = "io.grpc:${project.name}:${baselineGrpcVersion}@jar"
-            String depJar = "${project.name}-${baselineGrpcVersion}.jar"
-            Configuration configuration = configurations.detachedConfiguration(
-                    dependencies.create(depModule)
-                    )
-            baselineArtifact = files(configuration.files).filter {
-                it.name.equals(depJar)
-            }.singleFile
+            dependencies {
+                baselineArtifact "io.grpc:${project.name}:${baselineGrpcVersion}@jar"
+            }
         } finally {
             project.group = oldGroup
         }
 
         // Add a japicmp task that compares the current .jar with baseline .jar
         tasks.register("japicmp", me.champeau.gradle.japicmp.JapicmpTask) {
-            dependsOn jar
-            oldClasspath.from baselineArtifact
-            newClasspath.from jar.archiveFile
+            oldClasspath.from configurations.baselineArtifact
+            newClasspath.from tasks.named("jar")
             onlyBinaryIncompatibleModified = false
             // Be quiet about things that did not change
             onlyModified = true
@@ -454,12 +448,10 @@ subprojects {
 
             // Also break on source incompatible changes, not just binary.
             // Eg adding abstract method to public class.
-            // TODO(zpencer): enable after japicmp-gradle-plugin/pull/14
-            // breakOnSourceIncompatibility = true
+            failOnSourceIncompatibility = true
 
             // Ignore any classes or methods marked @ExperimentalApi
-            // TODO(zpencer): enable after japicmp-gradle-plugin/pull/15
-            // annotationExcludes = ['@io.grpc.ExperimentalApi']
+            annotationExcludes = ['@io.grpc.ExperimentalApi']
         }
     }
 }