WIP: Add a C++-implemented alternative to fixup_bundle

The fixup_bundle helper from the BundleUtilities module is meant to add runtime dependencies of a binary into its installation directory for use in producing redistributable packages. The current implementation in the CMake language is pretty complex, so it will be better to implement the functionality in C++.

Add a new undocumented file() command mode to hook into the C++ implementation from install scripts.