fix bug in fasd init

since we're sourcing this as a plugin, which loads the function, it's
counterproductive to also define the 'fasd' alias

moving the about-plugin metadata into the case block makes this script
safe for execution even if composure metadata isn't exported and
available

rename fasd.bash to fasd.plugin.bash to bring it inline with the other
plugin scripts

plugins/available/fasd.plugin.bash

@@ -584,7 +584,8 @@ fasd [-A|-D] [paths ...]
 fasd --init env
 
 case $- in
-  *i*) alias fasd=$BASH_IT'/plugins/enabled/fasd.bash'
+  *i*) cite about-plugin
+       about-plugin 'navigate "frecently" used files and directories'
        eval "$(fasd --init auto)"
       ;;
   *) # assume being executed as an executable