completion: Unset local variable outside condition block

Unset `curl_prefix` outside condition block so that it is always
cleared. Also, avoid `brew --prefix <foo>` since it is triggering ruby
in fallback flow when `<foo>` is not present.

Also, apply minor formatting tweaks.

modules/completion/init.zsh

@@ -12,14 +12,14 @@ if [[ $TERM == 'dumb' ]]; then
 fi
 
 # Add zsh-completions to $fpath.
-fpath=("${0:h}/external/src" $fpath)
+fpath=(${0:h}/external/src $fpath)
 
 # Add completion for keg-only brewed curl when available.
 if (( $+commands[brew] )) \
-      && [[ -d "${curl_prefix::="$(brew --prefix curl 2> /dev/null)"}" ]]; then
-  fpath=("$curl_prefix/share/zsh/site-functions" $fpath)
-  unset curl_prefix
+      && [[ -d "${curl_prefix::="$(brew --prefix 2> /dev/null)"/opt/curl}" ]]; then
+  fpath=($curl_prefix/share/zsh/site-functions $fpath)
 fi
+unset curl_prefix
 
 #
 # Options