Allow specifying a source file in files.*

This is in particular useful when the source file is generated with Nix.
Use for instance with:

```nickel
{
    files."foo".file = nix.import_nix "nixpkgs#foo",
}
```

lib/files.ncl

@@ -11,7 +11,18 @@ let File = {
     | doc m%"
         The content of the file.
       "%
-    | nix.derivation.NixString,
+    | nix.derivation.NullOr nix.derivation.NixString
+    | default
+    = null,
+  file
+    | doc "File from which to read the body of the script"
+    | nix.derivation.NullOr nix.derivation.NixString
+    | default
+    =
+      if content == null then
+        null
+      else
+        nix.builtins.to_file "generated-content" content,
   materialisation_method
     : [| 'Symlink, 'Copy |]
     | doc m%"
@@ -51,8 +62,11 @@ let regenerate_files | Files -> nix.derivation.Derivation = fun files_to_generat
         nix-s%"
             rm -f %{target}
             echo "Regenerating %{target}"
-	    target_dir=$(dirname %{target})
-	    test "${target_dir}" != "." && mkdir -p "${target_dir}"
+            target_dir=$(dirname %{target})
+            test "${target_dir}" != "." && mkdir -p "${target_dir}"
+            # XXX: If `source.file` is set explicitely to a relative path
+            # and `materialisation_method` is `'Symlink`, this will link to the
+            # original file, not one in the store. Not sure that's what we want.
             %{copy_command} %{file_in_store} %{target}
           "%
       in