Find version via hex_metadata.config instead of .hex

[kristoff3r]

The .hex file seem to be an outdated format, and it isn't present when building with Nix. You can easily get the same info using the hex_metadata.config file instead, which is described in the standard here: https://github.com/hexpm/specifications/blob/main/package_metadata.md.

I tested it locally by editing the mix.exs file in my deps folder and printing the version.

I'll just add that I think reading files and invoking git from a mix.exs as a library is bad practice, and I'd be happy to remove all of this logic as well if you want.