callCabal2nix: Fix calling with a path in the store.

lib/default.nix

@@ -93,7 +93,7 @@ let
       hiPrioSet;
     inherit (sources) pathType pathIsDirectory cleanSourceFilter
       cleanSource sourceByRegex sourceFilesBySuffices
-      commitIdFromGitRepo cleanSourceWith;
+      commitIdFromGitRepo cleanSourceWith pathHasContext canCleanSource;
     inherit (modules) evalModules closeModules unifyModuleSyntax
       applyIfFunction unpackSubmodule packSubmodule mergeModules
       mergeModules' mergeOptionDecls evalOptionValue mergeDefinitions

lib/sources.nix

@@ -93,4 +93,8 @@ rec {
                 else lib.head matchRef
            else throw ("Not a .git directory: " + path);
     in lib.flip readCommitFromFile "HEAD";
+
+  pathHasContext = builtins.hasContext or (lib.hasPrefix builtins.storeDir);
+
+  canCleanSource = src: src ? _isLibCleanSourceWith || !(pathHasContext (toString src));
 }