dbghelp: Add support for loading an external DWARF debug info file associated with a Mach-O module.
Such external debug info files are created by the dsymutil command. Usually, it
creates a <name>.dSYM bundle directory but it can also produce a <name>.dwarf
flat file. The code will find and use either. It first checks if such a bundle
or file is adjacent to the module binary. If it's not, it uses Spotlight to
search for a .dSYM bundle based on the binary's UUID, which is what gdb and lldb
do, too. That way, it can find it anywhere on the system.