Implement jar-in-jar loader system (#2899)

This fixes an issue that prevented LuckPerms from loading on Java 16

bukkit/loader/build.gradle

@@ -0,0 +1,33 @@
+plugins {
+    id 'com.github.johnrengelman.shadow'
+}
+
+repositories {
+    maven { url 'https://papermc.io/repo/repository/maven-public/' }
+}
+
+dependencies {
+    compileOnly 'com.destroystokyo.paper:paper-api:1.15.2-R0.1-SNAPSHOT'
+
+    compile project(':api')
+    compile project(':common:loader-utils')
+}
+
+processResources {
+    from(sourceSets.main.resources.srcDirs) {
+        expand 'pluginVersion': project.ext.fullVersion
+        include 'plugin.yml'
+    }
+}
+
+shadowJar {
+    archiveName = "LuckPerms-Bukkit-${project.ext.fullVersion}.jar"
+
+    from {
+        project(':bukkit').tasks.shadowJar.archiveFile
+    }
+}
+
+artifacts {
+    archives shadowJar
+}

bukkit/loader/src/main/java/me/lucko/luckperms/bukkit/loader/BukkitLoaderPlugin.java

@@ -0,0 +1,59 @@
+/*
+ * This file is part of LuckPerms, licensed under the MIT License.
+ *
+ *  Copyright (c) lucko (Luck) <luck@lucko.me>
+ *  Copyright (c) contributors
+ *
+ *  Permission is hereby granted, free of charge, to any person obtaining a copy
+ *  of this software and associated documentation files (the "Software"), to deal
+ *  in the Software without restriction, including without limitation the rights
+ *  to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+ *  copies of the Software, and to permit persons to whom the Software is
+ *  furnished to do so, subject to the following conditions:
+ *
+ *  The above copyright notice and this permission notice shall be included in all
+ *  copies or substantial portions of the Software.
+ *
+ *  THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+ *  IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+ *  FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+ *  AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+ *  LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+ *  OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
+ *  SOFTWARE.
+ */
+
+package me.lucko.luckperms.bukkit.loader;
+
+import me.lucko.luckperms.common.loader.JarInJarClassLoader;
+import me.lucko.luckperms.common.loader.LoaderBootstrap;
+
+import org.bukkit.plugin.java.JavaPlugin;
+
+public class BukkitLoaderPlugin extends JavaPlugin {
+    private static final String JAR_NAME = "luckperms-bukkit.jarinjar";
+    private static final String BOOTSTRAP_CLASS = "me.lucko.luckperms.bukkit.LPBukkitBootstrap";
+
+    private final LoaderBootstrap plugin;
+
+    public BukkitLoaderPlugin() {
+        JarInJarClassLoader loader = new JarInJarClassLoader(getClass().getClassLoader(), JAR_NAME);
+        this.plugin = loader.instantiatePlugin(BOOTSTRAP_CLASS, JavaPlugin.class, this);
+    }
+
+    @Override
+    public void onLoad() {
+        this.plugin.onLoad();
+    }
+
+    @Override
+    public void onEnable() {
+        this.plugin.onEnable();
+    }
+
+    @Override
+    public void onDisable() {
+        this.plugin.onDisable();
+    }
+
+}

bukkit/loader/src/main/resources/plugin.yml

@@ -0,0 +1,29 @@
+name: LuckPerms
+version: ${pluginVersion}
+description: A permissions plugin
+author: Luck
+website: https://luckperms.net
+main: me.lucko.luckperms.bukkit.loader.BukkitLoaderPlugin
+load: STARTUP
+
+# Mark the plugin as 1.13 compatible to avoid CB having to perform quite as much unnecessary
+# remapping when the plugin is loaded. Note that despite what this setting might otherwise imply,
+# LP is still compatible with pre-1.13 releases.
+api-version: 1.13
+
+# Load LuckPerms before Vault. This means that all plugins that (soft-)depend
+# on Vault depend on LuckPerms too.
+#
+# This fixes issues caused by plugins obtaining the Vault service provider instance
+# only once when they initially enable. (if they haven't depended on LP, our registration
+# won't be there yet)
+loadbefore: [Vault]
+
+# Soft depend on LilyPad for messaging service impl
+# Soft depend on ViaVersion for adventure protocol facet
+softdepend: [LilyPad-Connect, ViaVersion]
+
+commands:
+  luckperms:
+    description: Manage permissions
+    aliases: [lp, perm, perms, permission, permissions]