Fix compilable assets when using a mirrored public folder

When using a mirrored public folder (i.e. `artisan winter:mirror public`), the working directory for the request is set to base_path('public') instead of base_path(), so it is important to use absolute paths when calling File::exists() as it internally just calls file_exists(), which uses the current working directory to resolve relative paths.

Related:
- wintercms/storm@752c162#diff-58c1a5c5e22fc25e75bb02e4af9d2085cfe66b8cfe7c3cffc2f32a4fb60240aaR31
- laravel/framework#45679
- laravel/framework#13243

modules/system/classes/asset/PackageManager.php

@@ -307,7 +307,7 @@ public function registerPackage(string $name, string $path, string $type = 'mix'
         }
 
         // Check that the package path exists
-        if (!File::exists($path)) {
+        if (!File::exists(base_path($path))) {
             throw new SystemException(sprintf(
                 'Cannot register "%s" as a compilable package; the "%s" path does not exist.',
                 $name,
@@ -318,7 +318,7 @@ public function registerPackage(string $name, string $path, string $type = 'mix'
         $package = $path . '/package.json';
         $config = $path . DIRECTORY_SEPARATOR . $configFile;
 
-        if (!File::exists($config)) {
+        if (!File::exists(base_path($config))) {
             throw new SystemException(sprintf(
                 'Cannot register "%s" as a compilable package; the config file "%s" does not exist.',
                 $name,