Support both ninja python package and ninja_syntax + system ninja

[lllangWV]

Summary

Add fallback import for ninja_syntax to support both the ninja Python package and system-installed ninja, enabling conda-forge compatibility.

Motivation

I'm working to add spconv to conda-forge, which requires the following dependency chain:

• spconv → cumm → pccm → ccimport → ninja (python-ninja)

The ninja-python package on PyPI vendors its own compiled version of ninja, which violates conda-forge's policy against vendoring. This prevents ccimport from being added to conda-forge in its current form.

Changes

Modified the import statement to try importing from the ninja Python package first, then fall back to system ninja's ninja_syntax module:

try:
    from ninja.ninja_syntax import Writer
except ImportError:
    from ninja_syntax import Writer  # from system ninja

This allows ccimport to work with:

• The ninja Python package (ninja-python-distributions) on PyPI
• System-installed ninja (including conda-forge's ninja package)

Benefits

• Enables ccimport to be packaged for conda-forge
• Maintains backward compatibility with existing PyPI installations
• Follows the recommended pattern for ninja detection (similar to scikit-build-core's approach)
• Unblocks the conda-forge packaging of pccm, cumm, and ultimately spconv

Related Issues

This addresses the conda-forge policy requirement that packages should use system dependencies when available rather than vendored versions.