Versioned provides
As requested by Paul in #867081, here's a fix for making autopkgtest work with versioned Provides. There's some history in #761003. Just adding an extra 'apt-get install' call for binary packages listed in '@' is enough to fix the issue, and this renders the earlier fix for #761003 unnecessary so I've removed that in a separate commit.
I've tested this with a local version of the perl packages declaring versioned Provides, and it all seems to work correctly.
Please note that this probably introduces a slight behaviour change in install_apt() as the old code would afaics not verify that 'autopkgtest-satdep' stays installed if 'apt-get --fix-broken' had to be retried. I suspect the new behaviour is more desirable.
As you can see carrying the list of synthesized dependencies from _debian_packages_from_source() to install_apt() is not very clean in this patch; happy to test alternative implementations or whatever.
Many thanks for your work on Debian CI testing and greetings from the Debian Perl sprint + MiniDebConf in Hamburg!