Werk #15425: agent bakery: Change internal .deb format

This Werk fixes a little detail that will be irrelevant for most users.
It's best explained with an example.

Before:

$ ar x check_mk_agent.deb
$ tar tf control.tar.gz
postinst
preinst
prerm
control
conffiles
md5sums

Now:

$ ar x check_mk_agent.deb
$ tar tf control.tar.gz
.
./postinst
./preinst
./prerm
./control
./conffiles
./md5sums

While there's no advantage in packaging relative to ".", it seems to be the expected format for control.tar.gz and data.tar.gz, as we received some feedback about the SUSE manager not recognizing the .deb package otherwise.

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