Replace Afero with fs.FS

[sagikazarmark]

Is your feature request related to a problem? Please describe.
Afero is yet another dependency that Viper needs, but it's not really used to its full potential. I haven't seen anyone replacing the OS filesystem.

Describe the solution you'd like to see
Go 1.16 is going to ship a filesystem abstraction.

We should use it instead.

Describe alternatives you've considered
Come up with a simpler interface, owned by Viper, preferably modeling the new fs.FS interfaces..

Additional context
This would be a breaking change and would make Go 1.16 a hard requirement. While that might be an issue for some people, Viper 2 will actually make this easier: if #1046 (or a similar concept) gets implemented, we can provide an alternative file loader based on afero for those who need it.

[uruun]

Hi, I could take a look into this, although I didn't use fs.FS much yet. I think we can figure something out :). The first issue I can see is that fs.FS does not provide abstraction for writing to files. Another issue is that fstest.MapFS from stdlib that could be used in unit tests also has some limitations and I think would require some extension to work for viper. I can post a PR soon.

[sagikazarmark]

@uruun thanks, but I don’t think we can realistically get rid of Afero at this point.

As you also pointed out, write support is rather poor in io/fs.

Let’s hold off on this for now.