populate flake-input: fail on failures, reuse the store, leave out replaced inputs

Delivering flakes and flake inputs gets more robust:

- With fetchOnTarget, the inputs get collected before the pipeline that
  links them, so populate fails if the target can't fetch one, instead of
  linking the flake without its inputs and succeeding.
- What the lock file pins gets taken from the store if it's there: the
  store path is the one of a directory named source with the locked hash,
  which nix-store --print-fixed-path computes.  builtins.fetchTree
  contacts where the input comes from even then, so redeploying unchanged
  sources fails without that if a server is unreachable.
- With fetchOnTarget, a delivered input that is itself locked to a local
  path the target lacks fails with populate's error naming the input, like
  its inputs do.
- Inputs that get drawn from other sources are left out, with all they
  pin: Nix archives an empty flake in their place.  So nothing gets
  fetched, copied, and rooted that no evaluation uses, and a pin that is
  unreachable doesn't matter if it gets replaced anyway.
- flake-input sources get the option requireInputs.  By default,
  populating fails if the inputs that the input's own lock file pins
  can't be delivered, e.g. because the lock file lacks an input its
  flake.nix has.  With false, populate warns and delivers the input with
  what Nix locks now, or else without its inputs.

A remote flake that the target fetches entirely on its own collects its
inputs like everything else does.  The Nix function finding its
subdirectory moves next to the jq programs.

New edge cases cover each of these.

1 files changed, 4 insertions, 0 deletions

diff --git a/lib/types/populate.nix b/lib/types/populate.nix
index a278ffd..1a18b62 100644
--- a/lib/types/populate.nix
+++ b/lib/types/populate.nix
@@ -233,6 +233,10 @@
         name = lib.mkOption {
           type = lib.types.str;
         };
+        requireInputs = lib.mkOption {
+          default = true;
+          type = lib.types.bool;
+        };
       };
     };
     git = lib.types.submodule {