Integrate Webpack 5 watch mode from upstream

Integrates the changes from:
sysgears/webpack-virtual-modules#69

lib/virtual.js

@@ -22,6 +22,10 @@ function createWebpackData(result) {
 	}).bind({ result: result });
 }
 
+function getModulePath(filePath, compiler) {
+	return path.isAbsolute(filePath) ? filePath : path.join(compiler.context, filePath);
+}
+
 /**
  * @param {Compiler} compiler - the webpack compiler
  */
@@ -51,8 +55,6 @@ function VirtualModulesPlugin(compiler) {
 			this._virtualFiles[file] = { stats: stats, contents: contents };
 			setData(statStorage, file, createWebpackData(stats));
 			setData(fileStorage, file, createWebpackData(contents));
-			compiler.fileTimestamps instanceof Map &&
-				compiler.fileTimestamps.set(file, +stats.mtime);
 			var segments = file.split(/[\\/]/);
 			var count = segments.length - 1;
 			var minCount = segments[0] ? 1 : 0;
@@ -103,13 +105,6 @@ function VirtualModulesPlugin(compiler) {
 
 	const watchRunHook = (watcher, callback) => {
 		this._watcher = watcher.compiler || watcher;
-		const virtualFiles = compiler.inputFileSystem._virtualFiles;
-		if (virtualFiles) {
-			Object.keys(virtualFiles).forEach(function(file) {
-				compiler.fileTimestamps instanceof Map &&
-				compiler.fileTimestamps.set(file, +virtualFiles[file].stats.mtime);
-			});
-		}
 		callback();
 	};
 
@@ -145,6 +140,7 @@ VirtualModulesPlugin.prototype.writeModule = function(filePath, contents) {
 		ctime: time,
 		birthtime: time
 	});
+	var modulePath = getModulePath(filePath, this.compiler);
 
 	// When using the WatchIgnorePlugin (https://github.com/webpack/webpack/blob/52184b897f40c75560b3630e43ca642fcac7e2cf/lib/WatchIgnorePlugin.js),
 	// the original watchFileSystem is stored in `wfs`. The following "unwraps" the ignoring
@@ -155,6 +151,27 @@ VirtualModulesPlugin.prototype.writeModule = function(filePath, contents) {
 		finalWatchFileSystem = finalWatchFileSystem.wfs;
 	}
 	this.compiler.inputFileSystem._writeVirtualFile(filePath, stats, contents);
+	if (finalWatchFileSystem &&
+		(finalWatchFileSystem.watcher.fileWatchers.size ||
+			finalWatchFileSystem.watcher.fileWatchers.length)
+	) {
+		var fileWatchers = finalWatchFileSystem.watcher.fileWatchers instanceof Map ?
+			Array.from(finalWatchFileSystem.watcher.fileWatchers.values()) :
+			finalWatchFileSystem.watcher.fileWatchers;
+		fileWatchers.forEach(function(fileWatcher) {
+			if (fileWatcher.path === modulePath) {
+				delete fileWatcher.directoryWatcher._cachedTimeInfoEntries;
+				fileWatcher.directoryWatcher.setFileTime(
+					filePath,
+					time,
+					false,
+					false,
+					null
+				);
+				fileWatcher.emit("change", time, null);
+			}
+		});
+	}
 };
 
 function getData(storage, key) {