Pull in pep517 high-level interfaces into poetry codebase

[abn]

With the changes in pypa/pyproject-hooks#83 high-level interfaces from the pep517 library has now moved into an "example". This means that we can lo longer rely on these. We need to allow for these to work from within poetry for managed projects. This could allow us to replace the current subprocess call to execute the build.

Alternatively, an initial solution might to be rely on simplified scripts to emulate existing use case.

[abn]

Alternatively, we should also keep an eye on https://github.com/FFY00/python-build

[github-actions]

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