Unable to install mamba-ssm as a pypi dependency

[kszlim]

Checks

• I have checked that this issue has not already been reported.

• I have confirmed this bug exists on the latest version of pixi, using pixi --version.

Reproducible example

# setup base environment
pixi init --pyproject
pixi add mamba-ssm

Issue description

I'm unable to install mamba-ssm as a pypi dependency, it breaks even if i add pip as a conda dep and if i add packaging as a pypi dep.

pixi add --pypi mamba-ssm
  ⠂ default:linux-64     [00:00:00] resolving mamba-ssm==2.0.4                                                                                                                                                                                                  × failed to solve the pypi requirements of 'default' 'linux-64'
  ├─▶ failed to resolve pypi dependencies
  ├─▶ Failed to download and build `mamba-ssm==2.0.4`
  ├─▶ Failed to build: `mamba-ssm==2.0.4`
  ╰─▶ Build backend failed to determine extra requires with `build_wheel()` with exit status: 1
      --- stdout:
      
      --- stderr:
      Traceback (most recent call last):
        File "<string>", line 14, in <module>
        File "/path/to/home/.cache/rattler/cache/uv-cache/.tmpuEioZ7/.venv/lib/python3.12/site-packages/setuptools/build_meta.py", line 325, in get_requires_for_build_wheel
          return self._get_build_requires(config_settings, requirements=['wheel'])
                 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
        File "/path/to/home/.cache/rattler/cache/uv-cache/.tmpuEioZ7/.venv/lib/python3.12/site-packages/setuptools/build_meta.py", line 295, in _get_build_requires
          self.run_setup()
        File "/path/to/home/.cache/rattler/cache/uv-cache/.tmpuEioZ7/.venv/lib/python3.12/site-packages/setuptools/build_meta.py", line 487, in run_setup
          super().run_setup(setup_script=setup_script)
        File "/path/to/home/.cache/rattler/cache/uv-cache/.tmpuEioZ7/.venv/lib/python3.12/site-packages/setuptools/build_meta.py", line 311, in run_setup
          exec(code, locals())
        File "<string>", line 8, in <module>
      ModuleNotFoundError: No module named 'packaging'
      ---

I'm able to get it to install using pip on top of my pixi setup (but it required that i had a properly setup cuda/torch env + installed packaging), and also am able to get it installing with uv if I follow the workaround described here. I'm not sure what work would be required for it to work in pixi?

Expected behavior

Should by added with minimal fuss (and maybe an extra flag in the pyproject.toml if required)

[kszlim]

Looks like it related to #1124

[tdejager]

Indeed, if we support that flag we should be able to create a conda workspace with packaging and build without isolation using that dependency.

[tdejager]

Should now be possible with #1909 merged, and will soon be released, documentation is here: https://pixi.sh/dev/reference/project_configuration/#no-build-isolation