support/scripts: who's responsible to decide what is a package

When generating the package lists, the responsibility to decide what is
actually a package symbol is currently split between the _is_package(),
the get_symbol_subset() and the format_asciidoc_table() functions.

The two latter functions check that an item is really a symbol, and that
is has a prompt.

While this is currently correct for real packages, this will no longer
be the case when we also generate a list of virtual packages, since they
do not have a prompt.

Move the responsibility to verify that a symbol is indeed a package symbol
to _is_package(), so it's all in one place, and makes it easier to change
for virtual packages.

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Cc: Samuel Martin <s.martin49@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>