Only use all_features=true in sdist when local path dependencies exist

[Copilot]

Problem

The maturin sdist command was always setting all_features: true to ensure all optional path dependencies are packaged into the source distribution. However, this caused failures for projects with mutually exclusive features, as reported in #2673.

For example, a project using inkwell with multiple incompatible features would fail during sdist generation because enabling all features simultaneously created conflicting dependencies.

Solution

This PR modifies the sdist command to only enable all_features: true when the project actually has local path dependencies. The logic is:

• With path dependencies: Enable all_features: true (preserves current behavior)
• Without path dependencies: Use default feature selection (new behavior)

Changes

1. Added has_path_dependencies() function in src/source_distribution.rs:

   • Efficiently scans the dependency graph to detect local path dependencies
   • Returns true if any path dependencies are found, false otherwise

2. Modified SDist command in src/main.rs:

   • Checks for path dependencies before building
   • Conditionally sets all_features based on the result
   • Falls back to false if metadata cannot be retrieved

3. Added unit test to verify the path dependency detection works correctly for both scenarios

Testing

• ✅ Projects with path dependencies continue to work (existing behavior preserved)
• ✅ Projects without path dependencies now work with incompatible features
• ✅ All existing sdist tests pass
• ✅ New unit test verifies detection logic

Example

Before this fix, a project with incompatible features would fail:

$ maturin sdist
# Failed because all_features=true activated incompatible features

After this fix, the same project succeeds:

$ maturin sdist  
# Succeeds because no path dependencies detected, all_features=false

Projects with actual path dependencies continue to work as before, ensuring optional path dependencies are still properly packaged.

Fixes #2673

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