[MERGE] Make command discovery entirely callback driven.

[Robert Collins]

This makes the act of looking up a command entirely callback driven.

This enables several things:
 - it allows scripts that want to use bzrlib provide a UI which does not
   include the stock UI commands.
 - it makes the code more extensible - for instance, a command-name
corrector can be done without making the core code more clumsy
 - we can move to be even lazier about command object construction.

-Rob
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