Clean up usages / deprecate path helpers (#2155)

* Write source map without creating temp file

Less I/O, and one less place where we access the global path helpers.

* Drop useless app_path() helper

This was probably taken straight from Laravel. There is no equivalent
concept in Flarum, so this should be safe to remove.

* Deprecate global path helpers

Developers using these helpers can inject the `Paths` class instead.

* Stop storing paths as strings in container

* Avoid using path helpers from Application class

* Deprecate path helpers from Application class

* Avoid using public_path() in prerequisite check

a) The comparison was already outdated, as a different path was passed.
b) We're trying to get rid of these global helpers.

src/Locale/LocaleServiceProvider.php

@@ -11,6 +11,7 @@ namespace Flarum\Locale;
 
 use Flarum\Foundation\AbstractServiceProvider;
 use Flarum\Foundation\Event\ClearingCache;
+use Flarum\Foundation\Paths;
 use Flarum\Settings\SettingsRepositoryInterface;
 use Illuminate\Contracts\Events\Dispatcher;
 use Illuminate\Contracts\Translation\Translator as TranslatorContract;
@@ -73,6 +74,6 @@ class LocaleServiceProvider extends AbstractServiceProvider
 
     private function getCacheDir(): string
     {
-        return $this->app['flarum']->storagePath().'/locale';
+        return $this->app[Paths::class]->storage.'/locale';
     }
 }