Treat all symlinks in sources as to-be-built files

This means symlinks can never be ordinary inputs.

lib.nix

@@ -2,7 +2,7 @@
 let
   lib = pkgs.lib;
   inherit (builtins) baseNameOf dirOf length genList pathExists isString match
-  replaceStrings;
+  replaceStrings readDir;
   inherit (lib) take splitString last foldl foldr head tail removePrefix
   hasSuffix removeSuffix flatten crossLists reverseList all any unique hasPrefix
   cleanSourceWith;
@@ -99,7 +99,14 @@ let
 
       # Filter non-existant sources and build them with resolver.
       # Recreate the source directory structure.
-      nonExistantSrcs = builtins.filter (x: !builtins.pathExists x) srcs;
+      nonExistantSrcs = builtins.filter (x: let
+        isLink = x: let
+          base = baseNameOf x;
+          parent = dirOf x;
+          type = (readDir parent).${base} or null;
+        in (type == "symlink");
+      in !pathExists x || isLink x) srcs;
+
       builtSrcs = map (s: let
         relPath = removePrefix (root + "/") s;
         filePath = resolver s;
@@ -113,7 +120,7 @@ let
       srcFilter = path:
         if srcs == []
         then true
-        else (any (x: (hasPrefix x path)) srcs);
+        else (any (x: (hasPrefix x path)) (builtins.filter pathExists srcs));
 
       # Allow all directories lying on the way to specified srcs.
       _srcs = (augmentedSrcs root srcs);