Add safeguards against duplicate paths on the rtp

There was a bug where if you tried to maktaba#plugin#Install a plugin that was
already on the runtimepath as a RELATIVE path, then maktaba would also add the
ABSOLUTE version of the plugin path to the runtimepath. This would allow the
plugin to be sourced twice.

Fixing this involved adding a maktaba#rtp#Remove function.
Bump version to 1.1.0

Added the maktaba#rtp#Remove function.
Regenerated help docs.

Also, tweaked the wording in maktaba#rtp#Join() which was somewhat unclear.
Make rtp#Add and rtp#Remove respect the rtp cache

The cache was only being updated by rtp#Split.
typo fix: he -> the
Move rtp.vim constants outside of the resource-guard.