Avoid unnecessary rtp remove/re-add in some cases

`maktaba#plugin#Install` and related functions were sometimes removing and re-adding plugin paths on &runtimepath to force them to be the canonical path. This tends to shuffle the rtp ordering around (#49), has some quirks in the logic, and is really something that should be left up to the plugin manager instead of maktaba.

Now, maktaba will leave rtp alone in the cases that previously triggered it to remove/re-add a path, and it will only add paths if it can't detect that the path is already on rtp.