The 'all' arch makes it possible to not actually have all the
BPPH records for all archs, depending on how we've been called
previously (i.e. with which archs and/or no archs).
It's safer to just maintain our cache and recheck it each time if needed.
In https://code.launchpad.net/~cjwatson/launchpad/archive-unambiguous-files-traversals/+merge/345118,
I added support for downloading source package files from Launchpad that
can cope with the situation where different versions of a source package
have the same file name with different contents (normally impossible,
but it can happen with imported archives and due to some old bugs). Use
this where possible.