call URIStart in cdrom and file method

call URIStart in cdrom and file method

All other methods call it, so they should follow along even if the work
they do afterwards is hardly breathtaking and usually results in a
URIDone pretty soon, but the acquire system tells the individual item
about this via a virtual method call, so even through none of our
existing items contains any critical code in these, maybe one day they
might. Consistency at least once…

Which is also why this has a good sideeffect: file: and cdrom: requests
appear now in the 'apt-get update' output. Finally - it never made sense
to hide them for me. Okay, I guess it made before the new hit behavior,
but now that you can actually see the difference in an update it makes
sense to see if a file: repository changed or not as well.