fix: keep flake inputs' sourceInfo to flake attributes (#16)

[sini]

Summary

Fixes #16.

A flake input was built as sourceInfo // outputs // { … }, where sourceInfo is whatever the adapter handed over. nixtamal hands over fetchgit derivations, so every derivation attribute landed on the input: builder, drvAttrs, stdenv, type, fetchSubmodules and so on. That is the pollution in the report.

This change keeps only the attributes a real flake's sourceInfo can carry: outPath, narHash, rev, shortRev, dirtyRev, dirtyShortRev, revCount, lastModified, lastModifiedDate and submodules. It applies to flake inputs, default.nix-with-inputs inputs, and top-level self. If the adapter supplies rev, shortRev is derived from it, as Nix does. Source info is now merged over outputs, matching Nix's call-flake.nix and #23, so an output can't shadow outPath/rev.

Non-flake inputs (flake = false, or a source without flake.nix/default.nix) are unchanged and still expose the source as given.

Not in scope: whether narHash, rev and lastModified are present still depends on the adapter. tack's and npins' sources don't hand them over under those names (npins has revision/hash). So with tack you still won't see narHash/rev, but you no longer see anything a flake wouldn't have. Mapping each adapter's metadata onto flake names would be a follow-up per adapter.

Validation

• nix-unit ./tests.nix: 48/48. Three new source-info tests: fetcher attributes don't leak and sourceInfo is exactly the flake keys; rev/shortRev/lastModified reach the input; source info wins over an output named rev (new fixtures/shadow-flake). On main, 45/48: the new tests fail there.
• Real nixtamal template input (den): on main it carries builder drvAttrs stdenv type; with this change it carries none of them.
• Template loaders nixtamal, tack and npins evaluate nixosConfigurations.igloo…home.username to tux with this change.

[vic]

Thanks, @sini. You are awesome <3