This tool was mostly already compatible with copying
`libswift_Concurrency.dylib` based on whether or not the executables it
was scanning linked it. The only thing missing was that it can now end
up copying from different source directories, the older `swift-5.0`
directory, and the new `swift-5.5` directory.
This change allows users to pass `--source-libraries` multiple times
(or picks a default with both) in order to have it scan both directories
for any libraries the app links. The biggest part of this change is
instead of storing just the library name throughout this logic we now
store the actual discovered path, so that we don't have to loop through
all potential source directories each time we need to find the library,
only the first time.
Replace the use of a VLA with a `std::vector`. This applies a RAII to
the allocation, and the allocation is a single instance so the malloc
traffic is not a concern.
