Remove need for visible "lucet_module_data" symbol

[awortman-fastly]

For signing, we need to get at ModuleData of a compiled object, but at rest (so, without mapping through dlopen). This structure is reachable through SerializedModule but as mentioned in signature.rs it's kind of convoluted to get to that way, right now - object doesn't expose file offsets and we have to translate the virtual address of ModuleData to a file offset to read the bytes.

It would be nice to do that translation and not need to look up the bytes of ModuleData through a symbol, since that is the remaining dependence for lucet_module_data to be a visible symbol. This probably involves extending object to make file offsets visible.