Implements Version#dependencies_count

[tiegz]

we need a "have dependencies been fetched yet?" flag on the Version model. To that end:

• adds Version#dependencies_count, which will be nil if we haven't fetched its deps yet
  • (note that we have an existing #runtime_dependencies_count column)
• adds Version#set_dependencies_count method
• call Version#set_dependencies_count after we've fetched a Version's dependencies (whether or not there are zero)
• adds one_off:backfill_version_dependencies_count task to backfill this column in batches