Use a lightweight core library

[sdispater]

Pull Request Check List

• Added tests for changed code.
• Updated documentation for changed code.

This PR extracts the core utilities of Poetry (like the building of distributions) into a separate package (poetry-core) that is lightweight and without any declared dependency, the dependencies are vendored instead.

This will help address the issues mentioned in #1993.

Fixes #1993

[kasteph]

Is it possible to have core somehow namespaced under poetry so that we will instead have requires = ["poetry.core>=1.0.0"]? Using a dash in the package name can be quite confusing, for example, when do we use the dash and the underscore (in docs/pyproject.md, it requires poetry_core).

[pradyunsg]

Both of these are equivalent to PyPI - so “_-” won't matter to PyPI. You can use poetry_core instead of poetry-core and they'll refer to the same thing in the dependency specification.

That's the interoperability standard (PEP 508/440) and I expect poetry follows it. :)

[kasteph]

Ah okay, good to know, thanks!

[sdispater]

I would have preferred to use a namespace package but, as far as I know, it's not possible since poetry is already its own package.

It would require to no longer have the poetry package but multiple packages (like poetry-core and poetry-cli) so I don't think it's worth it for now.

However, if someone tells me there is a way to do it without big changes I'd be happy to change it.

[sdispater]

In the end, I managed to make the poetry namespace work, so poetry-core will provide the poetry.core package.

[github-actions]

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