Issue #2303777 by sun, jhedstrom: Fixed Allow drupal components to depend on...

Issue #2303777 by sun, jhedstrom: Fixed Allow drupal components to depend on other components outside Drupal.

core/lib/Drupal/Component/README.txt

 Drupal Components are independent libraries that do not depend on the rest of
-Drupal in order to function.  Components MAY depend on other Components, but
-that is discouraged. Components MAY NOT depend on any code that is not part of
-PHP itself or another Drupal Component.
+Drupal in order to function.
+
+Components MAY depend on other Drupal Components or external libraries/packages,
+but MUST NOT depend on any other Drupal code.
+
+In other words, only dependencies that can be specified in a composer.json file
+of the Component are acceptable dependencies.  Every Drupal Component presents a
+valid dependency, because it is assumed to contain a composer.json file (even
+if it may not exist yet).
 
 Each Component should be in its own namespace, and should be as self-contained
 as possible.  It should be possible to split a Component off to its own