Avoid to copy Logstash's classes into the uber jar

Start in build.gradle, especially the LOGSTASH_CORE_PATH dependency and jar packaging configuration. Run ./gradlew jar, then inspect build/libs/logstash-input-java_input_example-1.0.2.jar with jar tf; done means the uber jar no longer contains org/logstash classes.

Logstash information:

Please include the following information:

1. Logstash version (e.g. bin/logstash --version) 8.4.0
2. Logstash installation source (e.g. built from source, with a package manager: DEB/RPM, expanded from tar or zip archive, docker)
3. How is Logstash being run (e.g. as a service/service manager: systemd, upstart, etc. Via command line, docker/kubernetes)
4. How was the Logstash Plugin installed

JVM (e.g. java -version):

If the affected version of Logstash is 7.9 (or earlier), or if it is NOT using the bundled JDK or using the 'no-jdk' version in 7.10 (or higher), please provide the following information:

1. JVM version (java -version) (bundled)
2. JVM installation source (e.g. from the Operating System's package manager, from source, etc).
3. Value of the JAVA_HOME environment variable if set.

OS version (uname -a if on a Unix-like system):

Description of the problem including expected versus actual behavior:
The uber jar (and the gem) created contains also the Logstash classes. This could create weird problems when loaded inside a runtime Logstash, because the classloader could load a class from this jar and not from the runtime Logstash creating weird linkage errors.

Steps to reproduce:

1. Build the jar with ./gradlew jar, eventually fixing the build.gradle with:

diff --git a/build.gradle b/build.gradle
index 5f54ad3..4a85c25 100644
--- a/build.gradle
+++ b/build.gradle
@@ -49,7 +49,7 @@ dependencies {
    compileOnly 'org.apache.logging.log4j:log4j-api:2.17.0' // provided by Logstash
    compileOnly 'org.apache.logging.log4j:log4j-core:2.17.0' // provided by Logstash
 
-    implementation fileTree(dir: LOGSTASH_CORE_PATH, include: "**/logstash-core.jar")
+    implementation fileTree(dir: LOGSTASH_CORE_PATH, include: "**/lib/jars/logstash-core.jar")

    testImplementation 'junit:junit:4.12'
    testImplementation 'org.jruby:jruby-complete:9.2.20.1'

2. verify that the created jar contains classes from Logstash:

$ jar tf ./build/libs/logstash-input-java_input_example-1.0.2.jar
META-INF/
META-INF/MANIFEST.MF
org/
org/logstashplugins/
org/logstashplugins/JavaInputExample.class
org/logstash/
org/logstash/JavaVersionUtils.class
org/logstash/Rubyfier.class
org/logstash/Valuefier$Converter.class
org/logstash/FieldReference$IllegalSyntaxException.class
org/logstash/Timestamp.class
org/logstash/Javafier.class
org/logstash/execution/
org/logstash/execution/ConvergeResultExt$FailedActionExt.class
org/logstash/execution/QueueBatch.class
org/logstash/execution/PipelineReporterExt$SnapshotExt.class