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/types.nix

@@ -94,14 +94,16 @@ rec {
     # derivation is a reserved keyword.
     package = mkOptionType {
       name = "derivation";
-      check = isDerivation;
-      merge = mergeOneOption;
+      check = x: isDerivation x || isStorePath x;
+      merge = loc: defs:
+        let res = mergeOneOption loc defs;
+        in if isDerivation res then res else toDerivation res;
     };
 
     path = mkOptionType {
       name = "path";
       # Hacky: there is no ‘isPath’ primop.
-      check = x: builtins.unsafeDiscardStringContext (builtins.substring 0 1 (toString x)) == "/";
+      check = x: builtins.substring 0 1 (toString x) == "/";
       merge = mergeOneOption;
     };