new finder library based on afero

[sagikazarmark]

This PR introduces a new (experimental) finder implementation using Afero. It's covered by more tests and is generally expected to be more stable.

Why not use io/fs?

Viper requires Go 1.19 at this point and Afero exposes an fs.FS interface, so technically it would be possible.

Since Viper already uses Afero, we can't completely replace it without a breaking change. Also, fs.FS doesn't have any writing mechanisms, so file writing and directory creation would be missing (although the long-term plan is to get rid of those).

Lastly, fs.FS is not particularly easy to use on Windows (as mentioned in golang/go#44279).

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An alternative implementation can be found in #1624 using fs.FS. We may reuse it in the future if we decide to drop Afero and use fs.FS only.

TODO

• [x] locafero currently returns results in a lexical order, breaking path priority (see this comment for details)

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