Prioritise own classes where possible

This adds the server property `Paper.DisableClassPrioritization` to disable
prioritization of own classes for plugins' classloaders.

This value is by default not present, and this will therefore break any
plugins which abuse behaviour related to not using their own classes
while still loading their own. This is often an issue with failing to
relocate or shade properly, such as when shading plugin APIs like Vault.

A plugin's classloader will first look in the same jar as it is loading
in for a requested class, then load it. It does not re-use other
plugins' classes if it has the chance to avoid doing so.

If a class is not found in the same jar as it is loading for and it does
find it elsewhere, it will still choose the class elsewhere. This is
intended behaviour, as it will only prioritise classes it has in its own
jar, no other plugins' classes will be prioritised in any other order
than the one they were registered in.

The patch in general terms just loads the class in the plugin's jar
before it starts looking elsewhere for it.

Spigot-API-Patches/0207-Prioritise-own-classes-where-possible.patch

@@ -0,0 +1,61 @@
+From 0000000000000000000000000000000000000000 Mon Sep 17 00:00:00 2001
+From: Mariell Hoversholm <proximyst@proximyst.com>
+Date: Mon, 27 Apr 2020 12:31:59 +0200
+Subject: [PATCH] Prioritise own classes where possible
+
+This adds the server property `Paper.DisableClassPrioritization` to disable
+prioritization of own classes for plugins' classloaders.
+
+This value is by default not present, and this will therefore break any
+plugins which abuse behaviour related to not using their own classes
+while still loading their own. This is often an issue with failing to
+relocate or shade properly, such as when shading plugin APIs like Vault.
+
+A plugin's classloader will first look in the same jar as it is loading
+in for a requested class, then load it. It does not re-use other
+plugins' classes if it has the chance to avoid doing so.
+
+If a class is not found in the same jar as it is loading for and it does
+find it elsewhere, it will still choose the class elsewhere. This is
+intended behaviour, as it will only prioritise classes it has in its own
+jar, no other plugins' classes will be prioritised in any other order
+than the one they were registered in.
+
+The patch in general terms just loads the class in the plugin's jar
+before it starts looking elsewhere for it.
+
+diff --git a/src/main/java/org/bukkit/plugin/java/PluginClassLoader.java b/src/main/java/org/bukkit/plugin/java/PluginClassLoader.java
+index 3a02dbe9d183bc907dcce081d8338d5716ed5242..2abd140ce238c511c513179e360782ab7ecb53c8 100644
+--- a/src/main/java/org/bukkit/plugin/java/PluginClassLoader.java
++++ b/src/main/java/org/bukkit/plugin/java/PluginClassLoader.java
+@@ -29,6 +29,7 @@ import org.jetbrains.annotations.Nullable;
+  * A ClassLoader for plugins, to allow shared classes across multiple plugins
+  */
+ public final class PluginClassLoader extends URLClassLoader { // Spigot
++    private static final boolean DISABLE_CLASS_PRIORITIZATION = Boolean.getBoolean("Paper.DisableClassPrioritization"); // Paper
+     public JavaPlugin getPlugin() { return plugin; } // Spigot
+     private final JavaPluginLoader loader;
+     private final Map<String, Class<?>> classes = new ConcurrentHashMap<String, Class<?>>();
+@@ -107,7 +108,10 @@ public final class PluginClassLoader extends URLClassLoader { // Spigot
+         Class<?> result = classes.get(name);
+ 
+         if (result == null) {
+-            if (checkGlobal) {
++            String path = name.replace('.', '/').concat(".class"); // Paper - moved up
++            JarEntry entry = jar.getJarEntry(path); // Paper - moved up
++
++            if (checkGlobal && (entry == null || DISABLE_CLASS_PRIORITIZATION)) { // Paper - prioritise own classes if they exist
+                 result = loader.getClassByName(name);
+ 
+                 if (result != null) {
+@@ -129,9 +133,7 @@ public final class PluginClassLoader extends URLClassLoader { // Spigot
+             }
+ 
+             if (result == null) {
+-                String path = name.replace('.', '/').concat(".class");
+-                JarEntry entry = jar.getJarEntry(path);
+-
++                // Paper - move code here up
+                 if (entry != null) {
+                     byte[] classBytes;
+