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a) The comparison was already outdated, as a different path was passed.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.

framework/core/tests/unit/Install/Prerequisite/WritablePathsTest.php

@@ -0,0 +1,71 @@
+<?php
+
+/*
+ * This file is part of Flarum.
+ *
+ * For detailed copyright and license information, please view the
+ * LICENSE file that was distributed with this source code.
+ */
+
+namespace Flarum\Tests\unit\Install\Prerequisite;
+
+use Flarum\Install\Prerequisite\WritablePaths;
+use Flarum\Tests\unit\TestCase;
+
+class WritablePathsTest extends TestCase
+{
+    public function test_no_problems_when_all_directories_are_writable()
+    {
+        $writable = new WritablePaths([
+            __DIR__.'/../../../fixtures/writable_paths/writable',
+        ]);
+
+        $this->assertCount(0, $writable->problems());
+    }
+
+    public function test_paths_can_be_given_with_wildcard()
+    {
+        $writable = new WritablePaths([
+            __DIR__.'/../../../fixtures/writable_paths/writable/*',
+        ]);
+
+        $this->assertCount(0, $writable->problems());
+    }
+
+    public function test_problems_when_one_path_is_missing()
+    {
+        $writable = new WritablePaths([
+            __DIR__.'/../../../fixtures/writable_paths/missing',
+            __DIR__.'/../../../fixtures/writable_paths/writable',
+        ]);
+
+        $problems = $writable->problems();
+        $this->assertCount(1, $problems);
+        $this->assertRegExp(
+            "%^The .+/tests/fixtures/writable_paths/missing directory doesn't exist$%",
+            $problems[0]['message']
+        );
+        $this->assertEquals(
+            'This directory is necessary for the installation. Please create the folder.',
+            $problems[0]['detail']
+        );
+    }
+
+    public function test_problem_details_filter_out_wildcard()
+    {
+        $writable = new WritablePaths([
+            __DIR__.'/../../../fixtures/writable_paths/missing/*',
+        ]);
+
+        $problems = $writable->problems();
+        $this->assertCount(1, $problems);
+        $this->assertRegExp(
+            "%^The .+/tests/fixtures/writable_paths/missing directory doesn't exist$%",
+            $problems[0]['message']
+        );
+        $this->assertEquals(
+            'This directory is necessary for the installation. Please create the folder.',
+            $problems[0]['detail']
+        );
+    }
+}