ncmpcpp: Allow str type values for mpdMusicDir option (#3565)

The default value of `programs.ncmpcpp.mpdMusicDir` is taken from
`services.mpd.musicDirectory` if the mpd module is enabled, which has
type `either path str`. `programs.ncmpcpp.mpdMusicDir` did not accept
`str` values, though, so an error was raised when the default value was
used and `services.mpd.musicDirectory` was set to a value of type `str`.

This commit changes the type of `programs.ncmpcpp.mpdMusicDir` to also
accept `str` to reflect the type of `services.mpd.musicDirectory`.

Fixes #3560

tests/modules/programs/ncmpcpp-linux/ncmpcpp-issue-3560.nix

@@ -0,0 +1,25 @@
+{ pkgs, ... }:
+
+{
+  config = {
+    # Minimal config reproducing
+    # https://github.com/nix-community/home-manager/issues/3560
+    programs.ncmpcpp.enable = true;
+
+    services.mpd.enable = true;
+    services.mpd.musicDirectory = "~/music";
+
+    test.stubs = {
+      ncmpcpp = { };
+      mpd = { };
+    };
+
+    nmt.script = ''
+      assertFileContent \
+        home-files/.config/ncmpcpp/config \
+        ${./ncmpcpp-issue-3560-expected-config}
+
+      assertPathNotExists home-files/.config/ncmpcpp/bindings
+    '';
+  };
+}