Btw, love the idea of the main module being dependency-free + optional plugins!

I wonder if you've considered registering file parsers the way the database/sql package does it? Something like this in main.go:

import _ github.com/cristalhq/aconfig/aconfighcl

I get that importing with side effects is a bit awkward, but this way we could minimize the submodules API (no need to export anything). And the Go community is pretty familiar with this method because of the database plugins.

Just thinking out loud 😅

Regarding sql.Register(...) API it's not explicit, that's was my main (pain) point.

Defining file format per:

	FileDecoders: map[string]aconfig.FileDecoder{
		// from `aconfigyaml` submodule
		// see submodules in repo for more formats
		".yaml": aconfigyaml.New(),
	},

..makes everything easier to understand + gives a free configuration feature to support different file extensions (even somewhat weird like .cfg, .config)