Allow for cases where user might've aliased some programs

The $(type -p | head -1) construct expects to always get a path, so need to ignore such aliases with -P

Closes https://github.com/pyenv/pyenv/issues/1662

libexec/pyenv-versions

@@ -33,7 +33,7 @@ if ! enable -f "${BASH_SOURCE%/*}"/pyenv-realpath.dylib realpath 2>/dev/null; th
     exit 1
   fi
 
-  READLINK=$(type -p greadlink readlink | head -1)
+  READLINK=$(type -P greadlink readlink | head -1)
   if [ -z "$READLINK" ]; then
     echo "pyenv: cannot find readlink - are you missing GNU coreutils?" >&2
     exit 1