much faster fn_exists - output of retropie_packages is significantly improved

in my chroot dev environment 

testing with
```
time sudo ./retropie_packages.sh >/dev/null
```
old code:
```
real	0m14.872s
user	0m14.057s
sys	0m1.523s
```
new code:
```
real	0m0.895s
user	0m0.857s
sys	0m0.037s
```