Symbols in /usr/lib/debug aren't automatically loaded

See https://github.com/radare/radare2/issues/4888 for context.

Several linux systems split debug symbols from executables and installs them under /usr/lib/debug/ e.g. /usr/path/to/bin/cmd has symbols at /usr/lib/debug/usr/path/to/bin/cmd. radare2 1.6.0 doesn't appear to load these symbols. The symbols for calls are missing, attemping to seek, s, to a function fails, etc. On the other hand, gdb automatically loads the symbols from these files.