Fix: npm script(dev:package) is not working on Windows (#40566)

bin/packages/watch.js

@@ -59,7 +59,9 @@ function isDirectory( pathname ) {
 function isSourceFile( filename ) {
 	// Only run this regex on the relative path, otherwise we might run
 	// into some false positives when eg. the project directory contains `src`
-	const relativePath = path.relative( process.cwd(), filename );
+	const relativePath = path
+		.relative( process.cwd(), filename )
+		.replace( /\\/g, '/' );
 
 	return (
 		/\/src\/.+\.(js|json|scss|ts|tsx)$/.test( relativePath ) &&
@@ -98,6 +100,7 @@ function isWatchableFile( filename, skip ) {
 	// Recursive file watching is not available on a Linux-based OS. If this is the case,
 	// the watcher library falls back to watching changes in the subdirectories
 	// and passes the directories to this filter callback instead.
+
 	if ( isDirectory( filename ) ) {
 		return true;
 	}
@@ -117,8 +120,9 @@ function isWatchableFile( filename, skip ) {
 function getBuildFile( srcFile ) {
 	// Could just use string.replace, but the user might have the project
 	// checked out and nested under another src folder.
-	const packageDir = srcFile.substr( 0, srcFile.lastIndexOf( '/src/' ) );
-	const filePath = srcFile.substr( srcFile.lastIndexOf( '/src/' ) + 5 );
+	const srcDir = `${ path.sep }src${ path.sep }`;
+	const packageDir = srcFile.substr( 0, srcFile.lastIndexOf( srcDir ) );
+	const filePath = srcFile.substr( srcFile.lastIndexOf( srcDir ) + 5 );
 	return path.resolve( packageDir, 'build', filePath );
 }