updater: Clean up LoadSrcTgtVersion2().

Rename to LoadSourceBlocks() by moving the target blocks parsing part
into the caller. This allows detecting whether the target blocks have
already had the expected data before loading the source blocks. It
doesn't affect anything when applying an update package for the first
time, but it skips loading the unneeded source blocks when resuming an
update. It additionally avoids unnecessarily dumping the "corrupt"
source/stash blocks when resuming an update.

Bug: 33694730
Test: Apply an incremental update with the new updater.
Test: Resume an incremental update with the new updater.
Change-Id: I794fd0d1045be7b3b7f8619285dc0dade01398d0