Rename xbus to xswiftbus

Summary:
I wanted to rename xbus because its relation to //swift// is not immediately clear from the name.

xswiftbus seems reasonable.

Reviewers: #swift_pilot_client, kbasan

Reviewed By: kbasan

Subscribers: kbasan, jenkins

Tags: #swift_pilot_client

Differential Revision: https://dev.swift-project.org/D14

src/xswiftbus/xswiftbus.map

@@ -0,0 +1,10 @@
+{
+ global:
+  XPluginStart;
+  XPluginStop;
+  XPluginEnable;
+  XPluginDisable;
+  XPluginReceiveMessage;
+ local:
+   *;
+};