fix: avoid child compilation listen parent's plugin

As Html-webpack-plugin runs a child compiler, which initiated
with part of parent compiler's plugins by webpack. If we listen
on `compilation`, handlers in `webpack-split-by-path` will be
copy to the child compilation instance, which cases some unwanted
extraction.

So, we just listen to `this-compilation`.

See
https://github.com/webpack/webpack/blob/webpack-1/lib/Compiler.js#L327-L328
and
https://github.com/webpack/webpack/blob/webpack-1/lib/Compiler.js#L363-L364

index.js

@@ -71,7 +71,7 @@ SplitByPathPlugin.prototype.apply = function (compiler) {
   var ignoreChunks = this.ignoreChunks;
   var manifestName = this.manifest;
 
-  compiler.plugin('compilation', function (compilation) {
+  compiler.plugin('this-compilation', function (compilation) {
     var extraChunks = {};
 
     // Find the chunk which was already created by this bucket.
@@ -81,7 +81,6 @@ SplitByPathPlugin.prototype.apply = function (compiler) {
     }
 
     compilation.plugin('optimize-chunks', function (chunks) {
-      if(this.name) return
 
       var addChunk = this.addChunk.bind(this);