speed up aws plugin by eliminating(as much as possible) calls to brew

plugins/aws/aws.plugin.zsh

@@ -1,9 +1,24 @@
 _homebrew-installed() {
   type brew &> /dev/null
+  _xit=$?
+  if [ $_xit -eq 0 ];then
+        # ok , we have brew installed
+        # speculatively we check default brew prefix
+        if [ -h  /usr/local/opt/awscli ];then
+                _brew_prefix="/usr/local/opt/awscli"
+        else
+                # ok , it is not default prefix
+                # this call to brew is expensive ( about 400 ms ), so at least let's make it only once
+                _brew_prefix=$(brew --prefix awscli)
+        fi
+        return 0
+   else
+        return $_xit
+   fi
 }
 
 _awscli-homebrew-installed() {
-  brew list awscli &> /dev/null
+  [ -r $_brew_prefix/libexec/bin/aws_zsh_completer.sh ] &> /dev/null
 }
 
 export AWS_HOME=~/.aws
@@ -28,7 +43,7 @@ function aws_profiles {
 compctl -K aws_profiles asp
 
 if _homebrew-installed && _awscli-homebrew-installed ; then
-  _aws_zsh_completer_path=$(brew --prefix awscli)/libexec/bin/aws_zsh_completer.sh
+  _aws_zsh_completer_path=$_brew_prefix/libexec/bin/aws_zsh_completer.sh
 else
   _aws_zsh_completer_path=$(which aws_zsh_completer.sh)
 fi