Remove inconsistent error on missing subpackage path

The behavior of the code was slightly puzzling: if the package path did
not exist, we could log an error and continue, but if it existed but
was empty, we would error out. To be consistent, simply always error out.

source/dub/packagemanager.d

@@ -994,10 +994,6 @@ symlink_exit:
 				p.normalize();
 				enforce(!p.absolute, "Sub package paths must be sub paths of the parent package.");
 				auto path = pack.path ~ p;
-				if (!existsFile(path)) {
-					logError("Package %s declared a sub-package, definition file is missing: %s", pack.name, path.toNativeString());
-					continue;
-				}
 				sp = Package.load(path, NativePath.init, pack);
 			} else sp = new Package(spr.recipe, pack.path, pack);