fix-next(HMR): resolve correctly modulePath in hot loaders (#781)

Use `this.rootContext` instead of `this.context`.
For example in this [second-page](https://github.com/NativeScript/nativescript-dev-webpack/blob/master/demo/TypeScriptApp/app/views/second-page.xml) module:

---> `this.resourcePath` is /Users/vchimev/git/nativescript-dev-webpack/demo/TypeScriptApp/app/views/second-page.xml
---> `this.context` is /Users/vchimev/git/nativescript-dev-webpack/demo/TypeScriptApp/app/views
---> `this.rootContext` is /Users/vchimev/git/nativescript-dev-webpack/demo/TypeScriptApp/app
---> `modulePath` should be ./views/second-page.xml

Author: vchimev

Diff for: markup-hot-loader.js

@@ -2,6 +2,6 @@ const { reload } = require("./hot-loader-helper");
 
 module.exports = function (source) {
     const typeMarkup = "markup";
-    const modulePath = this.resourcePath.replace(this.context, ".");
+    const modulePath = this.resourcePath.replace(this.rootContext, ".");
     return `${source};${reload({ type: typeMarkup, module: modulePath })}`;
 };

Diff for: script-hot-loader.js

@@ -2,6 +2,6 @@ const { reload } = require("./hot-loader-helper");
 
 module.exports = function (source) {
     const typeScript = "script";
-    const modulePath = this.resourcePath.replace(this.context, ".");
+    const modulePath = this.resourcePath.replace(this.rootContext, ".");
     return `${source};${reload({ type: typeScript, module: modulePath })}`;
 };

Diff for: style-hot-loader.js

@@ -2,6 +2,6 @@ const { reload } = require("./hot-loader-helper");
 
 module.exports = function (source) {
     const typeStyle = "style";
-    const modulePath = this.resourcePath.replace(this.context, ".");
+    const modulePath = this.resourcePath.replace(this.rootContext, ".");
     return `${source};${reload({ type: typeStyle, module: modulePath })}`;
 };