[MERGE] Fix #296620 by allowing plugins instead of requiring them.

[Aaron Bentley]

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Aaron Bentley wrote:
> Robert Collins wrote:

>> It also means you don't have
>> to call a special magic method to let this work in your code.
> 
> I think that performing this at import time is more magic than a method
> would be.

I have discovered another problem with this magic import; it prevents us
from controlling the plugin path.  Specifically, it forces us to load
plugins from /usr/lib/...site-packages/.

These may not be appropriate plugins.  For example, some people have bzr
and bzrtools 1.9 installed, but are running the Launchpad test suite.
Launchpad has its own copy of bzr, currently at bzr 1.7.  bzr 1.7
doesn't support lazy command loading, so it's incompatible with bzrtools
1.9.  But no matter how you load bzrlib 1.7, it will set up a sys.path
that includes an inappropriate bzrtools.

This is the cause of bug #297158.

Aaron
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