use afero iofs wrapper instead of own

[aligator]

https://github.com/spf13/afero/releases/tag/v1.6.0

[aligator]

Tried that (https://github.com/aligator/GoFAT/tree/afero-1-6-0), but the io fs wrapper from afero has some compatibillity issues with the official go io fs test utility:
spf13/afero#292
spf13/afero#293