NEW: Allow “%$” prefix in Injector::get()

Injector::get() looks up services by name. In yaml config it can make
things clearer to prefix service names by %$, which is how they must
be prefixed when referencing nested services within service definitions.

This change means that any other system referencing services will
support an optional prefix without needing to specifically code support
in themselves.

src/Core/Injector/Injector.php

@@ -929,6 +929,11 @@ class Injector implements ContainerInterface
      */
     protected function getNamedService($name, $asSingleton = true, $constructorArgs = [])
     {
+        // Allow service names of the form "%$ServiceName"
+        if (substr($name, 0, 2) == '%$') {
+            $name = substr($name, 2);
+        }
+
         // Normalise service / args
         list($name, $constructorArgs) = $this->normaliseArguments($name, $constructorArgs);