Allow options with type "package" to be store paths

For example, this allows writing

  nix.package = /nix/store/786mlvhd17xvcp2r4jmmay6jj4wj6b7f-nix-1.10pre4206_896428c;

Also, document types.package in the manual.

lib/strings.nix

@@ -218,4 +218,9 @@ rec {
 
   # Format a number adding leading zeroes up to fixed width.
   fixedWidthNumber = width: n: fixedWidthString width "0" (toString n);
+
+
+  # Check whether a value is a store path.
+  isStorePath = x: builtins.substring 0 1 (toString x) == "/" && dirOf (builtins.toPath x) == builtins.storeDir;
+
 }