C API: (breaking) remove nix-flake-c global init

(cherry picked from commit 6a192ec0cd)

src/libflake/flake/flake-primops.cc

@@ -0,0 +1,59 @@
+#include "flake-primops.hh"
+#include "eval.hh"
+#include "flake.hh"
+#include "flakeref.hh"
+#include "settings.hh"
+
+namespace nix::flake::primops {
+
+PrimOp getFlake(const Settings & settings)
+{
+    auto prim_getFlake = [&settings](EvalState & state, const PosIdx pos, Value ** args, Value & v) {
+        std::string flakeRefS(
+            state.forceStringNoCtx(*args[0], pos, "while evaluating the argument passed to builtins.getFlake"));
+        auto flakeRef = nix::parseFlakeRef(state.fetchSettings, flakeRefS, {}, true);
+        if (state.settings.pureEval && !flakeRef.input.isLocked())
+            throw Error(
+                "cannot call 'getFlake' on unlocked flake reference '%s', at %s (use --impure to override)",
+                flakeRefS,
+                state.positions[pos]);
+
+        callFlake(
+            state,
+            lockFlake(
+                settings,
+                state,
+                flakeRef,
+                LockFlags{
+                    .updateLockFile = false,
+                    .writeLockFile = false,
+                    .useRegistries = !state.settings.pureEval && settings.useRegistries,
+                    .allowUnlocked = !state.settings.pureEval,
+                }),
+            v);
+    };
+
+    return PrimOp{
+        .name = "__getFlake",
+        .args = {"args"},
+        .doc = R"(
+          Fetch a flake from a flake reference, and return its output attributes and some metadata. For example:
+
+          ```nix
+          (builtins.getFlake "nix/55bc52401966fbffa525c574c14f67b00bc4fb3a").packages.x86_64-linux.nix
+          ```
+
+          Unless impure evaluation is allowed (`--impure`), the flake reference
+          must be "locked", e.g. contain a Git revision or content hash. An
+          example of an unlocked usage is:
+
+          ```nix
+          (builtins.getFlake "github:edolstra/dwarffs").rev
+          ```
+        )",
+        .fun = prim_getFlake,
+        .experimentalFeature = Xp::Flakes,
+    };
+}
+
+} // namespace nix::flake::primops