Avoid eagerly resolving the configuration

Calling task.source.isEmpty does some sort of eager resolution of
the configurations that were added to the task source. This in
turn can trigger other actions within Gradle and cause weird
behaviour (in our case, it caused other plugins to get applied
and eventually tripped a mutation guard.

The wire rootset code has a isEmpty flag it keeps which, according
to the comments, is there explicitly to avoid resolving the
configuration, so we can use that instead.

Author: staktrace

wire-gradle-plugin/src/main/kotlin/com/squareup/wire/gradle/WirePlugin.kt

@@ -186,13 +186,18 @@ class WirePlugin : Plugin<Project> {
         task.group = GROUP
         task.description = "Generate protobuf implementation for ${source.name}"
 
+        var addedSourcesDependencies = false
         // Flatten all the input files here. Changes to any of them will cause the task to re-run.
         for (rootSet in protoSourceProtoRootSets) {
           task.source(rootSet.configuration)
+          if (!rootSet.isEmpty) {
+            // Use the isEmpty flag to avoid resolving the configuration eagerly
+            addedSourcesDependencies = true
+          }
         }
         // We only want to add ProtoPath sources if we have other sources already. The WireTask
         // would otherwise run even through we have no sources.
-        if (!task.source.isEmpty) {
+        if (addedSourcesDependencies) {
           for (rootSet in protoPathProtoRootSets) {
             task.source(rootSet.configuration)
           }