Add build-system.requires to lock file

• I have searched the issues of this repo and believe that this is not a duplicate.
• I have searched the FAQ and general documentation and believe that my question is not already covered.

Feature Request

Problem

A notable thing missing from poetry.lock are build-systems used to build from sdists.

This is useful in efforts like poetry2nix that uses Poetry metadata.
Nix build environments are constructed in a much stricter way where we cannot dynamically install/load build-systems.
Currently users of this tool have to resort to manually adding overrides into Nix code to work around this missing metadata.

This issue has been reported many times over the years in various forms:

• https://github.com/nix-community/poetry2nix/issues/568
• https://github.com/nix-community/poetry2nix/issues/1435

And has generally been a large source of maintenance that could be completely automated away.

Adding this metadata also has other uses such as building tools that can statically inspect the build-time dependency graph.

Solution

One more possible entries in poetry.lock:

• build-system.requires

We only need to know build-system.requires & up front when constructing the build environment.
There is no need to add build-system.backend or build-system.backend-path.

The PyPi API does not have this metadata. It would need to be extracted from the sdist.

Note

Do note that this is not a suggestion to lock build systems, merely to add the contents of build-system.requires.

Start with the poetry.lock generation path and how Poetry reads package metadata from sdists; the issue notes that the PyPI API does not provide build-system.requires. Trace where sdist metadata can be extracted without adding backend or backend-path. Done means poetry.lock records build-system.requires for applicable packages and existing lock behavior remains intact.