Look for crates in LIBDIR, provided by --libdir option

Fixies #11671

This commit changes default relative libdir 'lib' to a relative libdir calculated using LIBDIR provided by --libdir configuration option. In case if no option was provided behavior does not change.

I don't think that rust will work by default if you pass `--libdir` (even with the changes here), so we may just want to completely remove the option.