Refactor Project to Gradle (#3720)

Gradle is better than Maven, don't @ me. okay but actually it's [faster](https://www.youtube.com/watch?v=atuFSv2bLa8&feature=youtu.be&t=77), compiles and tests in parallel more efficiently, and more epic stuff).

Essentials/src/main/java/com/earth2me/essentials/AlternativeCommandsHandler.java

@@ -0,0 +1,107 @@
+package com.earth2me.essentials;
+
+import org.bukkit.command.Command;
+import org.bukkit.command.PluginCommand;
+import org.bukkit.command.PluginCommandYamlParser;
+import org.bukkit.plugin.Plugin;
+
+import java.util.ArrayList;
+import java.util.HashMap;
+import java.util.Iterator;
+import java.util.List;
+import java.util.Locale;
+import java.util.Map;
+import java.util.logging.Level;
+import java.util.logging.Logger;
+
+public class AlternativeCommandsHandler {
+    private static final Logger LOGGER = Logger.getLogger("Essentials");
+    private final transient Map<String, List<PluginCommand>> altcommands = new HashMap<>();
+    private final transient Map<String, String> disabledList = new HashMap<>();
+    private final transient IEssentials ess;
+
+    public AlternativeCommandsHandler(final IEssentials ess) {
+        this.ess = ess;
+        for (final Plugin plugin : ess.getServer().getPluginManager().getPlugins()) {
+            if (plugin.isEnabled()) {
+                addPlugin(plugin);
+            }
+        }
+    }
+
+    public final void addPlugin(final Plugin plugin) {
+        if (plugin.getDescription().getMain().contains("com.earth2me.essentials")) {
+            return;
+        }
+        final List<Command> commands = PluginCommandYamlParser.parse(plugin);
+        final String pluginName = plugin.getDescription().getName().toLowerCase(Locale.ENGLISH);
+
+        for (final Command command : commands) {
+            final PluginCommand pc = (PluginCommand) command;
+            final List<String> labels = new ArrayList<>(pc.getAliases());
+            labels.add(pc.getName());
+
+            PluginCommand reg = ess.getServer().getPluginCommand(pluginName + ":" + pc.getName().toLowerCase(Locale.ENGLISH));
+            if (reg == null) {
+                reg = ess.getServer().getPluginCommand(pc.getName().toLowerCase(Locale.ENGLISH));
+            }
+            if (reg == null || !reg.getPlugin().equals(plugin)) {
+                continue;
+            }
+            for (final String label : labels) {
+                final List<PluginCommand> plugincommands = altcommands.computeIfAbsent(label.toLowerCase(Locale.ENGLISH), k -> new ArrayList<>());
+                boolean found = false;
+                for (final PluginCommand pc2 : plugincommands) {
+                    if (pc2.getPlugin().equals(plugin)) {
+                        found = true;
+                        break;
+                    }
+                }
+                if (!found) {
+                    plugincommands.add(reg);
+                }
+            }
+        }
+    }
+
+    public void removePlugin(final Plugin plugin) {
+        final Iterator<Map.Entry<String, List<PluginCommand>>> iterator = altcommands.entrySet().iterator();
+        while (iterator.hasNext()) {
+            final Map.Entry<String, List<PluginCommand>> entry = iterator.next();
+            entry.getValue().removeIf(pc -> pc.getPlugin() == null || pc.getPlugin().equals(plugin));
+            if (entry.getValue().isEmpty()) {
+                iterator.remove();
+            }
+        }
+    }
+
+    public PluginCommand getAlternative(final String label) {
+        final List<PluginCommand> commands = altcommands.get(label);
+        if (commands == null || commands.isEmpty()) {
+            return null;
+        }
+        if (commands.size() == 1) {
+            return commands.get(0);
+        }
+        // return the first command that is not an alias
+        for (final PluginCommand command : commands) {
+            if (command.getName().equalsIgnoreCase(label)) {
+                return command;
+            }
+        }
+        // return the first alias
+        return commands.get(0);
+    }
+
+    public void executed(final String label, final PluginCommand pc) {
+        final String altString = pc.getPlugin().getName() + ":" + pc.getLabel();
+        if (ess.getSettings().isDebug()) {
+            LOGGER.log(Level.INFO, "Essentials: Alternative command " + label + " found, using " + altString);
+        }
+        disabledList.put(label, altString);
+    }
+
+    public Map<String, String> disabledCommands() {
+        return disabledList;
+    }
+}