(Maybe) Fixes less#3187 (couldn't repo, but found bug)

- Fixes multiple Node.js plugins not being loaded

Gruntfile.js

@@ -190,7 +190,10 @@ module.exports = function (grunt) {
                 command: [
                     'node bin/lessc --clean-css="--s1 --advanced" test/less/lazy-eval.less tmp/lazy-eval.css',
                     'cd lib',
-                    'node ../bin/lessc --clean-css="--s1 --advanced" ../test/less/lazy-eval.less ../tmp/lazy-eval.css'
+                    'node ../bin/lessc --clean-css="--s1 --advanced" ../test/less/lazy-eval.less ../tmp/lazy-eval.css',
+                    'cd ..',
+                    // Test multiple plugins
+                    'node bin/lessc --plugin=clean-css="--s1 --advanced" --plugin=autoprefix="ie 11,Edge >= 13,Chrome >= 47,Firefox >= 45,iOS >= 9.2,Safari >= 9" test/less/lazy-eval.less tmp/lazy-eval.css'
                 ].join(' && ')
             },
             "sourcemap-test": {

lib/less-node/plugin-loader.js

@@ -7,8 +7,7 @@ var path = require("path"),
  */
 var PluginLoader = function(less) {
     this.less = less;
-    this.require = require;
-    this.requireRelative = function(prefix) {
+    this.require = function(prefix) {
         prefix = path.dirname(prefix);
         return function(id) {
             var str = id.substr(0, 2);
@@ -25,7 +24,6 @@ var PluginLoader = function(less) {
 PluginLoader.prototype = new AbstractPluginLoader();
 
 PluginLoader.prototype.loadPlugin = function(filename, basePath, context, environment, fileManager) {
-    var self = this;
     var prefix = filename.slice(0, 1);
     var explicit = prefix === "." || prefix === "/" || filename.slice(-3).toLowerCase() === ".js";
     if (!explicit) {
@@ -36,7 +34,6 @@ PluginLoader.prototype.loadPlugin = function(filename, basePath, context, enviro
         fileManager.loadFile(filename, basePath, context, environment).then(
             function(data) {
                 try {
-                    self.require = self.requireRelative(data.filename);
                     fulfill(data);
                 }
                 catch (e) {

lib/less/environment/abstract-plugin-loader.js

@@ -2,6 +2,10 @@ var functionRegistry = require("../functions/function-registry"),
     LessError = require('../less-error');
 
 var AbstractPluginLoader = function() {
+    // Implemented by Node.js plugin loader
+    this.require = function() {
+        return null;
+    }
 };
 
 function error(msg, type) {
@@ -63,7 +67,7 @@ AbstractPluginLoader.prototype.evalPlugin = function(contents, context, imports,
 
     try {
         loader = new Function("module", "require", "registerPlugin", "functions", "tree", "less", "fileInfo", contents);
-        loader(localModule, this.require, registerPlugin, registry, this.less.tree, this.less, fileInfo);
+        loader(localModule, this.require(filename), registerPlugin, registry, this.less.tree, this.less, fileInfo);
     } catch (e) {
         return new this.less.LessError(e, imports, filename);
     }