stack: accept the modern snapshot: key as well as resolver:

[hamishmack]

Stack 2.15.1 renamed the top-level resolver: key in stack.yaml to snapshot: (both remain accepted, and stack new now emits snapshot:). haskell.nix only recognised resolver:, so a project with a snapshot:-only stack.yaml fails during project evaluation with:

stack-repos: AesonException "Error in $: key \"resolver\" not found"
CallStack (from HasCallStack):
  error, called at lib/StackRepos.hs:36:15 ...

(see #2506).

Changes

• nix-tools/nix-tools/lib/Stack2nix/Stack.hs — accept snapshot as a synonym for resolver in both the FromJSON Stack and FromJSON StackSnapshot instances (s .: "resolver" <|> s .: "snapshot"). <|> is already imported and used by the Location instance.
• lib/fetch-resolver.nix — match ^snapshot: as well as ^resolver: when locating a resolver that points to another file.

Verification

• lib/fetch-resolver.nix: evaluated on a snapshot: lts-22.0 stack.yaml returns "lts-22.0"; a resolver: lts-21.0 one still returns "lts-21.0".
• Built stack-repos from this source and ran it:
  • snapshot: lts-22.0 → succeeds (repos.json produced) — previously crashed with key "resolver" not found.
  • resolver: lts-21.0 → still succeeds (no regression).

Fixes #2506.