When a Wi-Fi interface is present in the machine configuration (e.g.,
mwhudson.json), the GUI seemingly ignores it. This happens because there
is a filter on the server side which only returns Wi-Fi interfaces if
the wlan_support_install_state() function returns
PackageInstallState.DONE.

However, calling the /network endpoint shows that the state is set to
the wrong value:

 {"wlan_support_install_state": "NOT_NEEDED"}

This turns out to be inconsistent because:
 * we lean on a PackageInstaller instance to tell if wpasupplicant is
installed (this is what the wlan_support_install_state() function
reflects) ; but
 * in dry-run mode, we pretend to install wpasupplicant without
actually relying on the PackageInstaller instance.

Fixed by using the PackageInstaller instance to install the
wpasupplicant package - with a special implementation that only pretends
to install it. This is enough to make the PackageInstaller instance
think the package is installed.

Signed-off-by: Olivier Gayot <olivier.gayot@canonical.com>

Signed-off-by: Olivier Gayot <olivier.gayot@canonical.com>