Add limited SourceGraph module imports diagram, remove some imports

In my ongoing "high-level analysis" of Pontarius XMPP, I've proceeded
to take a quick look at the module imports.

The diagram provided is limited in that it does not include the
concurrent implementation, the Sasl modules, the IM modules, the XEP
modules, the example modules, nor the test modules.

Also, Network.Xmpp really does not need Network.Xmpp.Marshal,
Network.Xmpp.Stream, Network.Xmpp.Bind, or Network.Xmpp.Connection_.
Network.Xmpp.Connection does not need Network.Xmpp.Stream either.
These imports, and some others, have been removed with this patch.

Note also that Network.Xmpp.Tls does not need Network.Xmpp.Pickle.

Some observations:

1. There are three simple "utility" modules that depends only on
   Network.Xmpp.Types: Utilities, Presence, and Message. Utilities is
   inaccessible (indicated by the red colour).

2. There are two exposed "base" modules, Network.Xmpp and
   Network.Xmpp.Connection. The former does not need to access
   Network.Xmpp.Connection_, while the latter does. Otherwise, they
   both import the three modules: Session, Tls, and Types.
   Network.Xmpp also import the "utility" modules
   Network.Xmpp.Message, Network.Xmpp.Presence, and Network.Xmpp.Jid.

3. In Network.Xmpp.Session, if the `session' function is moved to the
   concurrent implementation, we can get rid of a dependency towards
   the concurrent implementation in the internal Network.Xmpp modules.
   If we take it one step further and also remove the Sasl functions,
   we can remove the need for Network.Xmpp to import this module (and
   probably get rid of the module completely).

4. Data.Conduit.Tls is only accessed by Network.Xmpp.Tls.

5. Text.Xml.Stream.Elements is only accessed by Network.Xmpp.Stream,
   Network.Xmpp.Connection_, and Network.Xmpp.Pickle.

6. Network.Xmpp.Types depends on Network.Xmpp.Jid (and re-exports it).