fix(__load_completion): do not warn when completing `.` first time

[scop]

ca361be02a92e30a3d26d376b30faf268853e836 enables warnings about trying to source directories when loading completions. That's useful, but triggers the errors in the common case of trying to source something, . Tab

Special case . (and for completeness, even though not that interesting, ..) so we don't issue the warning about them.