build: Properly define giomoduledir and session_bus_services_dir

Following the recommendations[0], the `giomoduledir` and
`session_bus_services_dir` use the information from the `pc` files
where they are defined, gio-2.0.pc and dbus-1.pc. However, these
variables are relative to some other variable which is set to be
a directory under prefix, where the user should be able to write.

On the other hand, systemd related paths, `systemduserunitdir`
and `tmpfilesdir` can not use this approach because they use
absolute paths, so there is no way to set those paths to
directories under prefix.

The `gio_module_dir` and `dbus_services_dir` have also been removed
because they are not necessary anymore.

https://bugzilla.gnome.org/show_bug.cgi?id=794365

[0] https://www.bassi.io/articles/2018/03/15/pkg-config-and-paths/