[5.6] Rework Logging (#22635)

This is a re-write / re-work of many of @phroggyy's ideas to improve logging.

Implemented:

Multi-driver logging support so you can have multiple logging channels per app.
"Pipes" in @phroggyy's pull request have been re-worked as "taps". Allows customization of logging channel post-creation.
Continued support for functionality provided by configureMonologUsing maintained using custom driver type with factory invokable that returns any PSR3 compatible logger. This allows full customization of entire logging channel.
I decided not to implement Log::event functionality from PR because I think that would be better suited for something like a ShouldLog interface on the event itself (to complement ShouldBroadcast, etc.).

Primary breaking changes:

New log configuration file / settings.
configureMonologUsing now custom driver type.
Illuminate\Contracts\Log removed. Was literal duplication of PSR3 logging interface.
Illuminate\Log\Writer renamed to Illuminate\Log\Logger.

src/Illuminate/Foundation/Application.php

@@ -1139,7 +1139,7 @@ public function registerCoreContainerAliases()
             'filesystem.cloud'     => [\Illuminate\Contracts\Filesystem\Cloud::class],
             'hash'                 => [\Illuminate\Hashing\HashManager::class],
             'translator'           => [\Illuminate\Translation\Translator::class, \Illuminate\Contracts\Translation\Translator::class],
-            'log'                  => [\Illuminate\Log\Writer::class, \Illuminate\Contracts\Logging\Log::class, \Psr\Log\LoggerInterface::class],
+            'log'                  => [\Illuminate\Log\Logger::class, \Illuminate\Contracts\Logging\Log::class, \Psr\Log\LoggerInterface::class],
             'mailer'               => [\Illuminate\Mail\Mailer::class, \Illuminate\Contracts\Mail\Mailer::class, \Illuminate\Contracts\Mail\MailQueue::class],
             'auth.password'        => [\Illuminate\Auth\Passwords\PasswordBrokerManager::class, \Illuminate\Contracts\Auth\PasswordBrokerFactory::class],
             'auth.password.broker' => [\Illuminate\Auth\Passwords\PasswordBroker::class, \Illuminate\Contracts\Auth\PasswordBroker::class],

src/Illuminate/Foundation/helpers.php

@@ -509,7 +509,7 @@ function info($message, $context = [])
      *
      * @param  string  $message
      * @param  array  $context
-     * @return \Illuminate\Contracts\Logging\Log|null
+     * @return \Illuminate\Log\LogManager|null
      */
     function logger($message = null, array $context = [])
     {
@@ -521,6 +521,19 @@ function logger($message = null, array $context = [])
     }
 }
 
+if (! function_exists('logs')) {
+    /**
+     * Get a log driver instance.
+     *
+     * @param  string  $driver
+     * @return \Illuminate\Log\LogManager|\Psr\Log\LoggerInterface
+     */
+    function logs($driver = null)
+    {
+        return $driver ? app('log')->driver($driver) : app('log');
+    }
+}
+
 if (! function_exists('method_field')) {
     /**
      * Generate a form field to spoof the HTTP verb used by forms.