debian/control: try installing virtualenv first, if it exists

debian/control: try installing virtualenv first, if it exists

Debian Jessie (at least) now has the virtualenv command in
package 'virtualenv', which depends on python3-virtualenv, and
that's how you get the virtualenv command there.  Earlier
Debian-based releases still use python-virtualenv.  Jessie's
virtualenv command still creates a python2 venv by default.

This will require a followup fix for install-deps.sh
to handle the | correctly, because currently it sends
the string to apt-get, which treats it as a regular expression
alternation of the two package names, so will try to install both.

The problem occurs when packages are installed without Recommended
packages (because jessie's virtualenv package currently Recommends
python-virtualenv); this is the case under pbuilder, and also using
'mk-build-deps --install', and I suspect under other automated
package-building tools.

Note that Build-Requires processing is not specified to
perform the searches in left-to-right order, so even this is
a tenuous workaround.  We probably need distro-specific
debian/ directories.

Signed-off-by: Dan Mick <dan.mick@redhat.com>