khard: add option to contact module for khard dir

Add an extra option,

    accounts.contact.accounts.<name>.khard.defaultCollection

to hardcode a subdirectory for khard to function as intended. Khard
expects to be given a collection directory directly, from which there
can be multiple of in a vdir. This contrasts khal or vdirsyncer which
support recursive search.

Fixes #4531

tests/modules/programs/khard/default.nix

@@ -2,4 +2,5 @@
   khard_empty_config = ./empty_config.nix;
   khard_basic_config = ./basic_config.nix;
   khard_multiple_accounts = ./multiple_accounts.nix;
+  khard_dirty_path = ./dirty_path.nix;
 }

tests/modules/programs/khard/dirty_path.nix

@@ -0,0 +1,22 @@
+{
+  accounts.contact = {
+    basePath = "/home/user/who/likes///";
+    accounts.forward = {
+      local.type = "filesystem";
+      khard = {
+        enable = true;
+        defaultCollection = "////slashes//a/lot";
+      };
+    };
+  };
+
+  programs.khard.enable = true;
+
+  test.stubs.khard = { };
+
+  nmt.script = ''
+    assertFileContent \
+      home-files/.config/khard/khard.conf \
+      ${./dirty_path_expected}
+  '';
+}

tests/modules/programs/khard/dirty_path_expected

@@ -0,0 +1,8 @@
+[addressbooks]
+[[forward]]
+path = /home/user/who/likes/forward/slashes/a/lot
+
+
+[general]
+default_action=list
+