Update lock files from InstallableFlake::toValue()

This ensures that the lock file is updated *before* evaluating it, and
that it gets updated for any nix command, not just 'nix build'.

Also, while computing the lock file, allow arbitrary registry lookups,
not just at top-level.

Also, improve some error messages slightly.

src/nix/command.hh

@@ -66,11 +66,6 @@ struct Installable
 
     Buildable toBuildable();
 
-    virtual std::optional<std::string> installableToFlakeUri()
-    {
-        return std::nullopt;
-    }
-
     virtual Value * toValue(EvalState & state)
     {
         throw Error("argument '%s' cannot be evaluated", what());
@@ -81,6 +76,8 @@ struct SourceExprCommand : virtual Args, StoreCommand, MixEvalArgs
 {
     std::optional<Path> file;
 
+    bool updateLockFile = true;
+
     SourceExprCommand();
 
     ref<EvalState> getEvalState();