Fix syncedFolder path when projectDirectory differs

[Ckitakishi]

Description

When the spec file and the generated project are in different directories, a syncedFolder source gets a path relative to the spec directory instead of the project directory. Xcode resolves that path from the xcodeproj location, so it points at a directory that doesn't exist. The synced folder is then silently treated as empty: the target compiles zero files but still reports BUILD SUCCEEDED.

# Modules/Shared/FooKit/project.yml
targets:
  FooKit:
    type: framework
    platform: iOS
    sources:
      - path: Sources
        type: syncedFolder

$ xcodegen generate --spec Modules/Shared/FooKit/project.yml --project .

# actual:   path = Sources;                        (resolves to <root>/Sources, doesn't exist)
# expected: path = Modules/Shared/FooKit/Sources;

The syncedFolder branch computes the relative path from project.basePath, which is the spec directory. Paths written into the pbxproj need to be relative to the output directory instead. Regular groups already handle this through resolveGroupPath, which uses the basePath property (projectDirectory ?? project.basePath).

Added a regression test that fails without the fix.