Move openEvalCache to libflake

Most of the eval cache logic is flake-independent and libexpr,
but the loading part is not.
`nix-flake` is the right component for this, as the eval cache
isn't exactly specific to the command line.

src/libcmd/include/nix/cmd/installable-flake.hh

@@ -87,6 +87,4 @@ static inline FlakeRef defaultNixpkgsFlakeRef()
     return FlakeRef::fromAttrs(fetchSettings, {{"type", "indirect"}, {"id", "nixpkgs"}});
 }
 
-ref<eval_cache::EvalCache> openEvalCache(EvalState & state, std::shared_ptr<flake::LockedFlake> lockedFlake);
-
 } // namespace nix

src/libcmd/installables.cc

@@ -443,42 +443,6 @@ static StorePath getDeriver(ref<Store> store, const Installable & i, const Store
     return *derivers.begin();
 }
 
-ref<eval_cache::EvalCache> openEvalCache(EvalState & state, std::shared_ptr<flake::LockedFlake> lockedFlake)
-{
-    auto fingerprint = evalSettings.useEvalCache && evalSettings.pureEval
-                           ? lockedFlake->getFingerprint(state.store, state.fetchSettings)
-                           : std::nullopt;
-    auto rootLoader = [&state, lockedFlake]() {
-        /* For testing whether the evaluation cache is
-           complete. */
-        if (getEnv("NIX_ALLOW_EVAL").value_or("1") == "0")
-            throw Error("not everything is cached, but evaluation is not allowed");
-
-        auto vFlake = state.allocValue();
-        flake::callFlake(state, *lockedFlake, *vFlake);
-
-        state.forceAttrs(*vFlake, noPos, "while parsing cached flake data");
-
-        auto aOutputs = vFlake->attrs()->get(state.symbols.create("outputs"));
-        assert(aOutputs);
-
-        return aOutputs->value;
-    };
-
-    if (fingerprint) {
-        auto search = state.evalCaches.find(fingerprint.value());
-        if (search == state.evalCaches.end()) {
-            search =
-                state.evalCaches
-                    .emplace(fingerprint.value(), make_ref<nix::eval_cache::EvalCache>(fingerprint, state, rootLoader))
-                    .first;
-        }
-        return search->second;
-    } else {
-        return make_ref<nix::eval_cache::EvalCache>(std::nullopt, state, rootLoader);
-    }
-}
-
 Installables SourceExprCommand::parseInstallables(ref<Store> store, std::vector<std::string> ss)
 {
     Installables result;

src/libflake/flake.cc

@@ -1,6 +1,9 @@
 #include "nix/util/terminal.hh"
+#include "nix/util/ref.hh"
+#include "nix/util/environment-variables.hh"
 #include "nix/flake/flake.hh"
 #include "nix/expr/eval.hh"
+#include "nix/expr/eval-cache.hh"
 #include "nix/expr/eval-settings.hh"
 #include "nix/flake/lockfile.hh"
 #include "nix/expr/primops.hh"
@@ -924,8 +927,6 @@ void callFlake(EvalState & state, const LockedFlake & lockedFlake, Value & vRes)
     state.callFunction(*vCallFlake, args, vRes, noPos);
 }
 
-} // namespace flake
-
 std::optional<Fingerprint> LockedFlake::getFingerprint(ref<Store> store, const fetchers::Settings & fetchSettings) const
 {
     if (lockFile.isUnlocked(fetchSettings))
@@ -953,4 +954,41 @@ std::optional<Fingerprint> LockedFlake::getFingerprint(ref<Store> store, const f
 
 Flake::~Flake() {}
 
+ref<eval_cache::EvalCache> openEvalCache(EvalState & state, std::shared_ptr<LockedFlake> lockedFlake)
+{
+    auto fingerprint = state.settings.useEvalCache && state.settings.pureEval
+                           ? lockedFlake->getFingerprint(state.store, state.fetchSettings)
+                           : std::nullopt;
+    auto rootLoader = [&state, lockedFlake]() {
+        /* For testing whether the evaluation cache is
+           complete. */
+        if (getEnv("NIX_ALLOW_EVAL").value_or("1") == "0")
+            throw Error("not everything is cached, but evaluation is not allowed");
+
+        auto vFlake = state.allocValue();
+        callFlake(state, *lockedFlake, *vFlake);
+
+        state.forceAttrs(*vFlake, noPos, "while parsing cached flake data");
+
+        auto aOutputs = vFlake->attrs()->get(state.symbols.create("outputs"));
+        assert(aOutputs);
+
+        return aOutputs->value;
+    };
+
+    if (fingerprint) {
+        auto search = state.evalCaches.find(fingerprint.value());
+        if (search == state.evalCaches.end()) {
+            search = state.evalCaches
+                         .emplace(fingerprint.value(), make_ref<eval_cache::EvalCache>(fingerprint, state, rootLoader))
+                         .first;
+        }
+        return search->second;
+    } else {
+        return make_ref<eval_cache::EvalCache>(std::nullopt, state, rootLoader);
+    }
+}
+
+} // namespace flake
+
 } // namespace nix

src/libflake/include/nix/flake/flake.hh

@@ -5,6 +5,7 @@
 #include "nix/flake/flakeref.hh"
 #include "nix/flake/lockfile.hh"
 #include "nix/expr/value.hh"
+#include "nix/expr/eval-cache.hh"
 
 namespace nix {
 
@@ -218,6 +219,11 @@ lockFlake(const Settings & settings, EvalState & state, const FlakeRef & flakeRe
 
 void callFlake(EvalState & state, const LockedFlake & lockedFlake, Value & v);
 
+/**
+ * Open an evaluation cache for a flake.
+ */
+ref<eval_cache::EvalCache> openEvalCache(EvalState & state, std::shared_ptr<LockedFlake> lockedFlake);
+
 } // namespace flake
 
 void emitTreeAttrs(