Should have a maktaba#plugin#Register that doesn't install

We have `maktaba#plugin#Install` to install a plugin on the runtimepath and register it with maktaba. But that's a fairly heavy operation that should usually be left to plugin managers, and in many cases we just want to make sure a plugin has been initialized with maktaba without affecting the runtimepath. For instance, both `maktaba#plugin#Detect` and `maktaba#plugin#Enter` potentially install the plugin on runtimepath, but in both cases we already know it's on runtimepath and `#Install` only adds the overhead of rtp manipulation and the potential for stomping on things (#49).

We should implement a `maktaba#plugin#Register({dir})` that's a more pure maktaba registration/init utility. Instead of installing plugins on runtimepath, it could either throw an error if it detects a plugin isn't installed or just not even check runtimepath.