remove circular dependency on export plugin

[dimbleby]

The circular dependency has been bothering me for a while, but #6441 was the nudge to do something about it.

poetry should not depend on the plugin: the plugin should depend on poetry.

This means that docs that talk about poetry export arguably don't belong here at all: but mostly they already say "this is actually provided by poetry-plugin-export" so I've just beefed that up slightly.

I'm about to find out what I've broken in the github workflows by removing this dependency...

[finswimmer]

I'm courious how this should work. IIRC removing the plugin essentially means removing the export command from the default installation. This is something we really like to do in the future, but for now this would be a breaking change.

[dimbleby]

Sure, people who want poetry export will need to install the plugin themselves.

If we want poetry export to be a default command: then move it back in-tree in this repository. But if we want it to be a plugin, then let's treat it as a plugin.

[dimbleby]

I can see we're going to need consensus here... somehow.

While we're thinking about disentangling these two projects, here are some more thoughts.

I'd be inclined to transfer the bits of export-specific code that linger in this repository over to the plugin entirely. That is: get_project_dependency_packages(), and its children in the call graph.

That would allow deleting the horrid cross-repository github workflow, making it much easier to make fixes to the export function.

It would also make it easier to add enhancements if the export plugin was more involved in walking the dependency tree eg python-poetry/poetry-plugin-export#91 would like access to information that is known inside that code.

To be clear, I don't intend to do any of that any time soon. But this seems as good a place as any to dump some notes.

[xylar]

I'm still in favor of eliminating this circular dependency but I just wanted to point to my comment here:
#6441 (comment)
It seems we can handle circular dependencies in conda packages, it's just a little awkward.

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[radoering]

After having merged #9547, I think there is consensus that we can move on here.

[dimbleby]

I've rebased, am happy to accept pull requests on my fork with tweaks / allow maintainers to push / have someone else follow up separately with further change