Follow existing pattern for generated resources

Follow the same pattern as includedAgentDir/includedJarFileTree:
- declare pomPropertiesDir and pomPropertiesFileTree near the top
- add dependsOn(pomPropertiesFileTree) to processResources
- declare main.resources.srcDir(pomPropertiesDir) in the sourceSets block
- register generatePomProperties (WriteProperties) near generateAgentJarIndex
- wire pomPropertiesFileTree.builtBy(generatePomProperties)

This avoids the need for a task-provider-mapped srcDir since WriteProperties
outputs a single file (not a directory), so the fileTree + builtBy approach
is the correct way to express the dependency.

Author: jandro996

dd-java-agent/build.gradle

@@ -19,8 +19,12 @@ configurations {
 def includedAgentDir = project.layout.buildDirectory.dir("generated/included")
 def includedJarFileTree = fileTree(includedAgentDir)
 
+def pomPropertiesDir = project.layout.buildDirectory.dir("generated/maven-metadata")
+def pomPropertiesFileTree = fileTree(pomPropertiesDir)
+
 tasks.named("processResources") {
   dependsOn(includedJarFileTree)
+  dependsOn(pomPropertiesFileTree)
 }
 
 sourceSets {
@@ -34,6 +38,7 @@ sourceSets {
     java.srcDirs "${project.projectDir}/src/main/java11"
   }
   main.resources.srcDir(includedAgentDir)
+  main.resources.srcDir(pomPropertiesDir)
 }
 
 def java6CompileTask = tasks.named("compileMain_java6Java") {
@@ -325,16 +330,12 @@ def generateAgentJarIndex = tasks.register('generateAgentJarIndex', JavaExec) {
 sourceSets.main.resources.srcDir(generateAgentJarIndex)
 
 def generatePomProperties = tasks.register('generatePomProperties', WriteProperties) {
-  destinationFile = project.layout.buildDirectory.file("generated/maven-metadata/META-INF/maven/com.datadoghq/dd-java-agent/pom.properties")
+  destinationFile = pomPropertiesDir.map { it.file("META-INF/maven/com.datadoghq/dd-java-agent/pom.properties") }
   property("groupId", "com.datadoghq")
   property("artifactId", "dd-java-agent")
   property("version", project.providers.provider { project.version.toString() })
 }
-sourceSets.main.resources.srcDir(
-  generatePomProperties.map {
-    project.layout.buildDirectory.dir("generated/maven-metadata").get()
-  }
-  )
+pomPropertiesFileTree.builtBy(generatePomProperties)
 
 subprojects { Project subProj ->
   // Don't need javadoc task run for internal projects.