Fallback to trying to find `libmonado` from the same directory as `library_path` if `MND_libmonado_path` not present

[jvyden]

Currently, Proton's PRESSURE_VESSEL_IMPORT_OPENXR_1_RUNTIMES=1 1 option is rewriting the active_runtime.json file in such a way that it strips out vendor-specific fields, including Monado's MND_libmonado_path field. ValveSoftware/steam-runtime#782

This causes the libmonado path to be missing or in the future, potentially invalid.

I brought this up to Valve in the above linked issue but since it's a bit of a tricky issue in the context of Proton's container, in the meantime this can be worked around from libmonado-rs's side by checking for libmonado in the same directory as library_path.

I can tackle this myself if desired.