support/dependencies/dependencies.sh: check for Python argparse module

The script support/scripts/check-uniq-files uses the argparse Python
module. In most recent Python versions (starting with 2.7), the
argparse module is part of the standard library, and we already check
for the availability of Python in
support/dependencies/dependencies.sh.

However, when running on an ancient distribution with Python 2.6, the
argparse module is not part of the Python standard library, but
available as an external module. Without this module, the build fails,
because check-uniq-files, which is used in target-finalize, fails to
run.

To avoid this failure, this commit adds a check in
support/dependencies/dependencies.sh to verify that the argparse
module is available.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>
Acked-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>