adapt brew,gem,pip plugin to new structure

plugins/brew/brew.plugin.zsh

 # add brew completion function to path
-fpath=($ZSH/functions/brew $fpath)
+fpath=($ZSH/plugins/brew $fpath)
 autoload -U compinit
 compinit -i

plugins/gem.plugin.zsh → plugins/gem/gem.plugin.zsh

 # add brew completion function to path
-fpath=($ZSH/functions/gem $fpath)
+fpath=($ZSH/plugins/gem $fpath)
 autoload -U compinit
 compinit -i

plugins/pip.plugin.zsh → plugins/pip/pip.plugin.zsh

 # add brew completion function to path
-fpath=($ZSH/functions/pip $fpath)
+fpath=($ZSH/plugins/pip $fpath)
 autoload -U compinit
 compinit -i