Fix duplicate SwiftPM C target public headers causing build failures

Generating projects from SwiftPM packages with C targets that use directory-style module maps no longer fails with a “Multiple commands produce” build error. When a package defines both a top-level wrapper header and nested module headers sharing the same basename such as spm_headers/pb.h and spm_headers/nanopb/pb.h, Tuist previously emitted duplicate copy commands for <Framework>.framework/Headers/<basename>, breaking the build. Duplicates are now resolved so that nested module headers take precedence and only one header per basename lands in the framework bundle.