Reimplement lib.types.path.check

[roberth]

Describe the bug

• it adds source paths to the store types: Fix path.check for cleanSourceWith output #64691
• it is partial lib/types: Allow paths as submodule values #76861 (comment)

To Reproduce
Steps to reproduce the behavior:

Partiality:

nix-repl> lib.types.path.check {}
error: cannot coerce a set to a string, at /nix/var/nix/profiles/per-user/root/channels/nixos/lib/types.nix:246:42

Adding sources to store:

1. write a module with a types.path option
2. make the module use that path by further filtering the source
3. use it
4. intermediate source will be in the store (nb can be huge)

Expected behavior

No side effects, so also no exceptions.

Additional context

path.check will need to understand when string coercion is possible to avoid both exceptions and adding sources to the store.

toString implementation:
https://github.com/NixOS/nix/blob/f800d450b78091835ab7ca67847d76e75d877a24/src/libexpr/primops.cc#L1762

string coercion:
https://github.com/NixOS/nix/blob/f800d450b78091835ab7ca67847d76e75d877a24/src/libexpr/eval.cc#L1593-L1648

Metadata

Maintainer information:

# a list of nixpkgs attributes affected by the problem
attribute:
# a list of nixos modules affected by the problem
module: