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a) The comparison was already outdated, as a different path was passed.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/Install/InstallServiceProvider.php

@@ -23,15 +23,6 @@ class InstallServiceProvider extends AbstractServiceProvider
         $this->app->singleton('flarum.install.routes', function () {
             return new RouteCollection;
         });
-
-        $this->app->singleton(Installation::class, function () {
-            return new Installation(
-                $this->app->basePath(),
-                $this->app->publicPath(),
-                $this->app->storagePath(),
-                $this->app->vendorPath()
-            );
-        });
     }
 
     /**