[MERGE] Avoid unnecessarily resolving lazy objects in Registry.get_prefix().

[Jelmer Vernooij]

This patch makes Registry.get_prefix() not load objects until it has found a 
key that matches. Registry.get_prefix() is used by the directory services code,
which because of this currently loads all registered directory services, even 
if they are not used. 

On my system, this means not having to always load some parts of the launchpad 
and builddeb plugins including their dependencies (xmlrpc, apt_pkg).

Cheers,

Jelmer
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