Refactor SSH commands into their own package

This separates the command implementations from the supporting code
used by the rest of the daemon, thereby hiding most of the daemon's
implementation details from the commands.

Signed-off-by: Shawn O. Pearce <sop@google.com>

src/main/java/com/google/gerrit/server/ssh/NoShell.java

@@ -14,6 +14,8 @@
 
 package com.google.gerrit.server.ssh;
 
+import com.google.gerrit.server.SshServlet;
+
 import org.apache.sshd.server.ShellFactory;
 import org.spearce.jgit.lib.Constants;