Propagate project-level conflicts to package extras (#18096)
Closes #18015
Project-level conflict items (e.g., `{ package = "pkg-a" }`) were not
properly excluding the package's extras and groups from the conflicting
fork. When a project-level conflict excludes a package, all of that
package's extras should also be excluded (since they transitively depend
on the base package). The alternative seems to be that the user needs to
enumerate all of the extra conflicts explicitly in addition to the
package conflict, which seems excessive.
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Co-authored-by: Claude <noreply@anthropic.com>
