Improving support for ${target_name} expansion

I have found there are times when it is helpful to have some variable expansion. In particular, I have a postBuildScripts that I'd like to execute. In this case it is copying some files and would be great if I could use ${target_name} in this case to expand the name to use the target name so I can cut and paste this into files.

Right now it doesn't do anything, so it remains ${target_name}.

This is what I'd like to ideally do:

    postBuildScripts:
      - script: |
          cp ${DERIVED_FILES_DIR}/${target_name}-Swift.h ${BUILT_PRODUCTS_DIR}/${target_name}.framework/Headers
          cp -rv ${BUILT_PRODUCTS_DIR}/${target_name}.swiftmodule ${BUILT_PRODUCTS_DIR}/${target_name}.framework/Modules
        name: Copy SWIFT_OBJC_INTERFACE_HEADER_NAME and swiftmodule   

Note that I have improved the above script by assigning the target name to a single variable first and then using that internally. But it will still be a nice to have.

Start by tracing how XcodeGen handles the postBuildScripts entry and how the generated script receives variables. Use the example in the issue to check whether ${target_name} is expanded in post-build commands, and consider the work complete when the shown copy commands receive the target name instead of the literal placeholder.