files: allow a wider range of source file names

In particular support source files whose name start with `.` or
contain characters not allowed in the nix store, such as spaces.

Also add some test cases for `home.file`.

modules/lib/file-type.nix

@@ -4,27 +4,7 @@ with lib;
 
 let
 
-  # Figures out a valid Nix store name for the given path.
-  storeFileName = path:
-    let
-      # All characters that are considered safe. Note "-" is not
-      # included to avoid "-" followed by digit being interpreted as a
-      # version.
-      safeChars =
-        [ "+" "." "_" "?" "=" ]
-        ++ lowerChars
-        ++ upperChars
-        ++ stringToCharacters "0123456789";
-
-      empties = l: genList (x: "") (length l);
-
-      unsafeInName = stringToCharacters (
-        replaceStrings safeChars (empties safeChars) path
-      );
-
-      safeName = replaceStrings unsafeInName (empties unsafeInName) path;
-    in
-      "home_file_" + safeName;
+  stringsExtra = import ./strings.nix { inherit lib; };
 
 in
 
@@ -113,7 +93,7 @@ in
         source = mkIf (config.text != null) (
           mkDefault (pkgs.writeTextFile {
             inherit (config) executable text;
-            name = storeFileName name;
+            name = stringsExtra.storeFileName name;
           })
         );
       };