936061: get more pkg reports

Bugs like 936061 and 544812 indicate that reportbug could be improved by learning to query bugs across multiple packages at the same time. This MR implements that.

When retrieving the list to allow the user to check for existing bugs, so far we had to decide between querying bugs for either a binary package or a source package. This meant that bug relations like 'affects' could not be considered very well, and there was a risk of not retrieving many potentially relevant bugs.

To solve the problem, reportbug.debbugs.get_reports() is taught to accept a list of packages, retrieve bugs lists for all of them (also taking 'affects' into account), and query the bug summaries for the union of the retrieved bug lists. Reportbug is modified to call ui.handle_bts_query() (and thereby debbugs.get_reports) with a package list of binary and source package by default. Submit-As is also included if applicable.