This more intuitively matches types.either and allows paths to be coerced to submodules again.

Motivation for this change

#76861's changes allowed strings to typecheck as submodules when they appear to be a path (start with a slash). A somewhat common use of coercedTo is to turn single values into an attrset/submodule: with types; coercedTo str (value: { inherit value; }) valueSubmodule. coercedTo currently will prioritise the final type if possible, and now that submodules can be set from paths this causes havoc when the module system tries to import what was previously meant to be just a config value.

A similar issue occurs with types.either, and #76861 suggests changing the order of the types as a solution. That approach isn't possible here and coercedTo feels backward in comparison, so this change reverses its behaviour.

cc @infinisil who made the original submodule change and suggested this patch

Things done

• Tested using sandboxing (nix.useSandbox on NixOS, or option sandbox in nix.conf on non-NixOS linux)
• Built on platform(s)
  • NixOS
  • macOS
  • other Linux distributions
• Tested via one or more NixOS test(s) if existing and applicable for the change (look inside nixos/tests)
• Tested compilation of all pkgs that depend on this change using nix-shell -p nixpkgs-review --run "nixpkgs-review wip"
• Tested execution of all binary files (usually in ./result/bin/)
• Determined the impact on package closure size (by running nix path-info -S before and after)
• Ensured that relevant documentation is up to date
• Fits CONTRIBUTING.md.