This option provides a more convenient way to overlay dummy packages.
It also adds a function `config.lib.test.mkStubPackage` that can,
e.g., be used for `package` options.
This change makes the services created via the lieer module aware of the notmuch config created by the home-mangager notmuch module (which is stored in a non-standard location).
Without this change all the lieer services created by the lieer module failed for me, as they were unable to find the notmuch config.
Add 'services.lieer', which generates systemd timer and service units
to synchronize a Gmail account with lieer. Per-account configuration
lives in 'accounts.email.accounts.<name>.lieer.sync'.
