ensure sources that claim to have arch:all binaries have them

Until now, britney2 ignored the information in Sources about which binaries a package builds. Due to bugs elsewhere, we have a long standing bug 887060, which is about sources migrating because the arch:all build failed. Checking if arch:all binaries are present isn't that difficult, so let's ensure that if a source package mentions arch:all, it can only migrate if it has any arch:all binaries.

To support archives where the Sources is not to be relied on for the above check and where the admins might want to skip the safe-guard, a new option skipping the check. The same option will enable reporting on new arch:all binaries appearing (which are ignored under default operation) at the price that it prevents binNMU's from migrating in some corner case (see for more discussion bug 1064428).

Closes: #887060 Wontfix: #1064427, #1064428