If pullfile is None, then assume that there are no overrides to be
considered (the set of overrides is now the empty set). This is useful
in the case of Launchpad queue handling, where overrides do not make
sense.
This makes the method a little cleaner, as it no longer works out a
thing _and_ has a side effect. Instead, make it do a thing only and put
the side effect explicitly in __init__.
usd import-ppa: add new subcommand that imports a PPA's publishing history
Currently push is disabled, as we need to determine what the correct
behavior is for that.
Launchpad has a publishinghistory for packages in a PPA, it's just not
visible on the web UI.
Also, old source packages are deleted once they are no longer available
from the PPA on a periodic basis. So for a 'full history' of a PPA, you
have to run `usd import-ppa` on a regular basis and start it before the
first upload to the PPA (or along with the upload).
usd/source_information: add support for obtaining PPA publishing history
If a USDSourceInformation 'dist_name' starts with 'ppa:', we will treat
it special as a reference to a PPA, not a typical Ubuntu/Debian
publishing history request.
No callers yet use 'ppa:', so this should have no functional impact.