refactor: [build-system.build-backend]

[baszalmstra]

Description

We decided to refactor the way we format the [build-system] section because it was confusing, inconsistently formatted with the rest of the file and contained some duplication.

Previously the [build-system] section would look like:

[build-system]
dependencies = ["python-build-backend > 12"]
build-backend = "python-build-backend"
channels = ["conda-forge"]

We decided to reformat this as:

[build-system]
build-backend = { name = "python-build-backend", version = ">12"  }
channels = ["conda-forge"]

# optional
additional-dependencies = { git = "*" }

The build-backend describes both the package and the executable to invoke in the package. Additional dependencies can be added through the additional-dependencies field. I also added loads of errors for corner cases.

[wolfv]

Channels is also optional and defaults to the project channels, right?

[baszalmstra]

Yes but not in this pr yet

[wolfv]

@baszalmstra you might have to change it here as well: https://github.com/prefix-dev/pixi/blob/main/tests/integration_python/pixi_build/test-data/rattler-build-backend/pixi/pixi.toml