systemd.bbclass: do not automatically add Also services

systemd.bbclass: do not automatically add Also services

Starting a systemd service “Also” does not mean that both services
must be in the same package. However, the systemd.bbclass enforces
this.

Example:
  a.service:
    [Install]
    Also=b.service
If a.service is packed in package A, b.service is automatically packed
into package A as well. This happens even if b.service is explicitly
added to package B using FILES and SYSTEMD_SERVICE variables.

The automatic packing of socket files with the corresponding service
files is probably a widely used feature of systemd.bbclass. This bahavior
does not change.

Adding regular service files to a package just because it is another
service in the same package that "Also" uses the service is a bug that
this commit fixes.

Signed-off-by: Adrian Freihofer <adrian.freihofer@siemens.com>
Signed-off-by: Mathieu Dubois-Briand <mathieu.dubois-briand@bootlin.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>