Added Gradle's vendoring task already present in other mixed Java/Ruby plugins (#320)

This PR is to keep the plugin aligned to others that uses a mix of Ruby and Java, and to provide standard task sets.

With this PR, after vendored the plugin with

./gradle vendor

the plugin is usable directly in development mode, setting :path in Gemfile

Author: andsel

build.gradle

@@ -17,6 +17,9 @@
  * under the License.
  */
 
+import java.nio.file.Files
+import static java.nio.file.StandardCopyOption.REPLACE_EXISTING
+
 apply plugin: "java"
 apply plugin: "distribution"
 apply plugin: "idea"
@@ -28,7 +31,8 @@ repositories {
     mavenCentral()
 }
 
-project.sourceCompatibility = 1.8
+project.sourceCompatibility = JavaVersion.VERSION_1_8
+project.targetCompatibility = JavaVersion.VERSION_1_8
 
 dependencies {
     compileOnly group: 'org.jruby', name: 'jruby-complete', version: "9.1.13.0"
@@ -59,3 +63,37 @@ task cleanGemjar {
 
 clean.dependsOn(cleanGemjar)
 jar.finalizedBy(copyGemjar)
+
+
+task generateGemJarRequiresFile {
+    doLast {
+        File jars_file = file('lib/logstash-input-file_jars.rb')
+        jars_file.newWriter().withWriter { w ->
+            w << "# AUTOGENERATED BY THE GRADLE SCRIPT. DO NOT EDIT.\n\n"
+            w << "require \'jar_dependencies\'\n"
+            configurations.runtimeClasspath.allDependencies.each {
+                w << "require_jar(\'${it.group}\', \'${it.name}\', \'${it.version}\')\n"
+            }
+            w << "\nrequire_jar(\'${project.group}\', \'${project.name}\', \'${project.version}\')\n"
+        }
+    }
+}
+
+task vendor {
+    doLast {
+        String vendorPathPrefix = "vendor/jar-dependencies"
+        configurations.runtimeClasspath.allDependencies.each { dep ->
+            File f = configurations.runtimeClasspath.filter { it.absolutePath.contains("${dep.group}/${dep.name}/${dep.version}") }.singleFile
+            String groupPath = dep.group.replaceAll('\\.', '/')
+            File newJarFile = file("${vendorPathPrefix}/${groupPath}/${dep.name}/${dep.version}/${dep.name}-${dep.version}.jar")
+            newJarFile.mkdirs()
+            Files.copy(f.toPath(), newJarFile.toPath(), REPLACE_EXISTING)
+        }
+        String projectGroupPath = project.group.replaceAll('\\.', '/')
+        File projectJarFile = file("${vendorPathPrefix}/${projectGroupPath}/${project.name}/${project.version}/${project.name}-${project.version}.jar")
+        projectJarFile.mkdirs()
+        Files.copy(file("$buildDir/libs/${project.name}-${project.version}.jar").toPath(), projectJarFile.toPath(), REPLACE_EXISTING)
+    }
+}
+
+vendor.dependsOn(jar, generateGemJarRequiresFile)