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/strings.nix

@@ -0,0 +1,27 @@
+{ lib }:
+
+with lib;
+
+{
+  # 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
+      "hm_" + safeName;
+}