`AssemblyOp::location` is "dangling" when executed on another machine

If we create a `Package` from a program that contains debug information, and execute it on a different machine, then the `Location` will not resolve since the source file is not present on the other machine (or at least it's unlikely to be at the same path).

Is the `Package` format meant to be "release" mode only? If so, we could output a warning when creating a package from a `Program` with debug symbols present, and similarly output a warning when trying to execute a `Package` in debug mode.

Otherwise, we would need to add the ability to insert the source files in the `Package`, and when "installing" or "unboxing" the package on the destination machine, make sure one way or another that the `Location`s now point to the correct path.

cc @bitwalker @greenhat