Add detection for plugins manually added to runtimepath

Updates `maktaba#plugin#Get()` and `maktaba#plugin#IsRegistered()` to find plugins manually added to runtimepath (and plugins installed by a plugin manager that doesn't integrate tightly with maktaba). Also affects the behavior of `maktaba#library#Import()` and `#library#Require()`.

Fixes #37.

I didn't do any special caching yet for `&rtp` paths like I mentioned in #37 because any performance regressions will be fairly limited: this code is only run when maktaba would have otherwise complained that the plugin was NotFound, or when `#plugin#IsRegistered()` would have returned 0. The caching will probably become more of an issue in implementing #38.