Allow requesting a custom report using the report name or FQN

This allows for external standards to provide portable custom reports while installed in an arbitrary directory.

* The external standard has to make sure it includes an `<autoload>` directive in the ruleset and sort out the loading of the custom report classes through a `spl_autoload_register()`-ed autoloader.
* The name of the custom report class can be provided either on the command line using `--report=...` or via the ruleset using `<arg name="report" value=".."/>`.
* The name of the custom report class can be either the Fully Qualified report class name òr just the class name.
    - The FQN does not have to be prefixed with an `\`, but things will work just fine if it is.
    - If just the class name is provided, PHPCS will try to find the report in any of the namespaces as registered by the installed standards.
    - If the report class is not found in any of these, a `DeepExitException` will be thrown.

Fixes 1942

autoload.php

@@ -217,6 +217,18 @@ public static function addSearchPath($path, $nsPrefix='')
         }//end addSearchPath()
 
 
+        /**
+         * Retrieve the namespaces and paths registered by external standards.
+         *
+         * @return array
+         */
+        public static function getSearchPaths()
+        {
+            return self::$searchPaths;
+
+        }//end getSearchPaths()
+
+
         /**
          * Gets the class name for the given file path.
          *

src/Reporter.php

@@ -105,6 +105,7 @@ public function __construct(Config $config)
                 $output = $config->reportFile;
             }
 
+            $reportClassName = '';
             if (strpos($type, '.') !== false) {
                 // This is a path to a custom report class.
                 $filename = realpath($type);
@@ -114,8 +115,32 @@ public function __construct(Config $config)
                 }
 
                 $reportClassName = Autoload::loadFile($filename);
-            } else {
+            } else if (class_exists('PHP_CodeSniffer\Reports\\'.$type) === true) {
+                // PHPCS native report.
                 $reportClassName = 'PHP_CodeSniffer\Reports\\'.$type;
+            } else if (class_exists($type) === true) {
+                // FQN of a custom report.
+                $reportClassName = $type;
+            } else {
+                // OK, so not a FQN, try and find the report using the registered namespaces.
+                $registeredNamespaces = Autoload::getSearchPaths();
+                $trimmedType          = ltrim($type, '\\');
+
+                foreach ($registeredNamespaces as $nsPrefix) {
+                    if ($nsPrefix === '') {
+                        continue;
+                    }
+
+                    if (class_exists($nsPrefix.'\\'.$trimmedType) === true) {
+                        $reportClassName = $nsPrefix.'\\'.$trimmedType;
+                        break;
+                    }
+                }
+            }//end if
+
+            if ($reportClassName === '') {
+                $error = "ERROR: Class file for report \"$type\" not found".PHP_EOL;
+                throw new DeepExitException($error, 3);
             }
 
             $reportClass = new $reportClassName();