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

[Aaron Bentley]

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It's misleading to say "allowing plugins instead of requiring them."
Bzrtools is not required for the shelf UI to run.

The bug, IMHO, is that --no-plugins does not prevent plugins from loading.

Requiring bzrlib to never import from a plugin, even when there's a
fallback, makes bzrlib too fragile.

Vincent Ladeuil wrote:
> Not extra fanycness, but get the job done: restore test suite usability.
> 
> I don't really know how to proceed for the bzrtools related patch, so
> I'll attach it to the bug report.

Because the bug is also a bzr bug, my mail client filtered it into bzr
bugs and it got lost in the noise.

Generally, people submit patches directly to me, but you can also submit
a merge request on this mailing list and Bundle Buggy will do the right
thing.

Aaron
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