Get accurate progress reporting in apt update again

For the non-pdiff case, we have can have accurate progress
reporting because after fetching the {,In}Release files we know
how many IndexFiles will be fetched and what size they have.

Therefore init the filesize early (in pkgAcqIndex::Init) and
ensure that in Acquire::Pulse() looks at already downloaded
bits when calculating the progress in Acquire::Pulse.

Also improve debug output of Debug::acquire::progress

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