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a) The comparison was already outdated, as a different path was passed.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/Frontend/FrontendServiceProvider.php

@@ -10,6 +10,7 @@
 namespace Flarum\Frontend;
 
 use Flarum\Foundation\AbstractServiceProvider;
+use Flarum\Foundation\Paths;
 use Flarum\Frontend\Compiler\Source\SourceCollector;
 use Flarum\Http\UrlGenerator;
 use Flarum\Settings\SettingsRepositoryInterface;
@@ -21,14 +22,16 @@ class FrontendServiceProvider extends AbstractServiceProvider
     {
         $this->app->singleton('flarum.assets.factory', function () {
             return function (string $name) {
+                $paths = $this->app[Paths::class];
+
                 $assets = new Assets(
                     $name,
                     $this->app->make('filesystem')->disk('flarum-assets'),
-                    $this->app['flarum']->storagePath()
+                    $paths->storage
                 );
 
                 $assets->setLessImportDirs([
-                    $this->app['flarum']->vendorPath().'/components/font-awesome/less' => ''
+                    $paths->vendor.'/components/font-awesome/less' => ''
                 ]);
 
                 $assets->css([$this, 'addBaseCss']);