Cleaner buildable folder generation and cross-target file membership

Projects that use buildable folders now generate cleaner pbxproj output. Files inside a buildable folder, such as xcconfigs, Info.plist files, and entitlements, no longer create stray root-level file references. Xcconfigs inside a buildable folder use Xcode 16-style anchored references, and Info.plist/entitlements are surfaced through build settings only. Additionally, a new .exception(target:included:) manifest API lets you compile a file from one target’s buildable folder into another target without creating duplicate references.