build.script: use build environment for execution

[abn]

With this change, Poetry now creates an ephemeral build environment
with all requirements specified under build-system.requires when a
build script is specified. Otherwise, project environment is reused.

Resolves: #2789
Supercedes: python-poetry/poetry-core#58

Decided to solve this at the poetry level as we do not yet have consensus on if the build-system requirements should be considered as a project's development dependendency.

Alternative approaches

• build-system.requires is loaded into a new implicit dependency group callded build-system, this will mean that these will affect dependency resolution. However, this will also allow for the easier development of build scripts.

Future Enhancements

• The build environment is cached within the project's virtual environment. This will require Poetry to handle entire life-cycle if the dependencies change etc. This might also cause issues with system environments. However, this will also provide speedups.
• Reuse Poetry's internal wheel fetch logic instead of relying on pip.

[yajo]

This doesn't solve the problem of buidl-system.requires not being locked in poetry.lock, does it?

IMHO they should be locked there too, or we can't really have reproducible builds.

[abn]

@yajo unfortunately this PR is not intended to solve that.

Additionally, there is also the added issue that the build time dependency is managed by the PEP 517 frontends. The short version is, even if the lockfile contained your build system depdnencies your builds will not be reproducible across all cases. The only case it covers if you used poetry build. The only way to achieve reproducible builds is to lock the version in your pyproject.toml build-system.requires. Obviously, this will have to be manually managed.

[ghost]

note for future searchers: poetry 1.2.0 broke our previously successful builds (where we use a custom build = 'script.py' setting) until we added setuptools manually as:

[build-system]
requires = ["poetry-core", "setuptools"]

[github-actions]

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