Make index tasks cacheable and improve input fingerprinting (#11896)

chore(build): Make `agentJarIndex` task cacheable

Also, it improves the way gradle fingerprints the input

The class path is already declared as an input if the JavaExec spec, and
doesn't need an additinal declaration.

The included dir is `dd-java-agent/build/generated/included`
we don't really care about `included/` or where it is, but the task
cares about what's below, like `inst/foo/Bar.classdata`. So the idea is
to tell gradle to apply that normalization for fingerprinting.

https://docs.gradle.org/current/javadoc/org/gradle/api/tasks/PathSensitivity.html#RELATIVE

Additional note, that in CI the build dir of projects is relocated under

```
..../workspace/dd-java-agent/build
```

chore(build): Make instrumentation index tasks cacheable

The class path is already declared as an input if the JavaExec spec, and
doesn't need an additinal declaration.

Co-authored-by: brice.dutheil <brice.dutheil@datadoghq.com>

Author: bric3

dd-java-agent/build.gradle

@@ -348,8 +348,11 @@ def generateAgentJarIndex = tasks.register('generateAgentJarIndex', JavaExec) {
   it.mainClass = 'datadog.trace.bootstrap.AgentJarIndex$IndexGenerator'
 
   it.inputs.files(includedJarFileTree)
-  it.inputs.files(it.classpath)
+    .withPropertyName("includedAgentFiles")
+    .withPathSensitivity(PathSensitivity.RELATIVE)
   it.outputs.dir(destinationDir)
+    .withPropertyName("agentJarIndex")
+  it.outputs.cacheIf { true }
   it.classpath = objects.fileCollection().tap {
     it.from(project.configurations.named("shadowInclude"))
     it.from(project.configurations.named('slf4j-simple'))

dd-java-agent/instrumentation/build.gradle

@@ -153,8 +153,9 @@ TaskProvider<JavaExec> registerIndexTask(String indexTaskName, String indexer, S
       it.from(project.configurations.named("runtimeClasspath"))
       it.from(project.configurations.named('slf4j-simple'))
     }
-    it.inputs.files(it.classpath)
     it.outputs.dir(destinationDir)
+      .withPropertyName("indexDirectory")
+    it.outputs.cacheIf { true }
     it.argumentProviders.add(new CommandLineArgumentProvider() {
         @Override
         Iterable<String> asArguments() {