Removing mentions of $PATH, using zsh $path instead

In ZSH, there is an array variable called `$path` that mirrors the bash `$PATH`, and zsh automatically exports the `$path` to `$PATH` when initializing the environment. Several plugins `export $PATH` directly instead of adding to the `$path` variable, which makes things like `typeset -U path` to remove duplicates from the `$PATH` impossible. This patch moves all mentions of `$PATH` to `$path`.