Create fat builds of static, dynamic & framework libraries if both iOS
and simulator versions have been created from separate manual Xcode builds.
Refactor scripts for creating fat libraries to reuse across projects.
Add MVK_BUILT_PROD_DIR to replace use of BUILT_PRODUCTS_DIR in most scripts
to allow flexibility across per-platform compilation.
Create fat builds of external libraries if both iOS and simulator versions
have been created from separate manual Xcode builds.
Add Scripts/create_fat_libs.sh.
Remove Scripts/package_ext_libs_ios.sh and Scripts/package_ext_libs_macos.sh.