move Linux implementation to sub directory

Author: philenius

lib/src/file_picker.dart

@@ -2,9 +2,9 @@ import 'dart:async';
 import 'dart:io';
 
 import 'package:file_picker/src/file_picker_io.dart';
-import 'package:file_picker/src/file_picker_linux.dart';
 import 'package:file_picker/src/file_picker_macos.dart';
 import 'package:file_picker/src/file_picker_result.dart';
+import 'package:file_picker/src/linux/file_picker_linux.dart';
 import 'package:file_picker/src/windows/stub.dart'
     if (dart.library.io) 'package:file_picker/src/windows/file_picker_windows.dart';
 import 'package:plugin_platform_interface/plugin_platform_interface.dart';