Call apt-cache show once with list of packages

[jacobperron]

Attempt to resolve #554.

Instead of checking for virtual packages individually, now the entire list of packages to check are passed to apt-cache show. In my tests involving 58 uninstalled packages, this change (in combination with #556) has resulted in a speed up of 3 minutes.

Version	Time
0.11.8	3m13.276s
this commit	7m13.761s
#556	3m7.361s
this commit + #556	0m8.088s

It is important to note that I have not had the opportunity to explicitly test with actual virtual packages (ie. the for-loop at L144 never executes since virtual_packages has been empty in my test cases).

[mathias-luedtke]

This looks reasonable, but it will conflict with #521 (as-is).
@jacobperron: Can you please benchmark #521 as well?

[jacobperron]

@ipa-mdl I ran the same benchmark test for #521 with the result 0m26.544s.

[mathias-luedtke]

Thanks, this looks promising!
However, apt-cache showpkg can display for one package only.
Together with #556 it should be fast enough :D