This will tell Arborist to save an appropriate lockfile based on
the version specified.

Version 3 = packages section only
Version 2 = packages and dependencies sections (default)
Version 1 = dependencies section only (legacy npm v6 support)

Re: npm/rfcs#434

If lockfileVersion is explicitly set to 3, and then later it is not
explicitly set, then we continue to use a version 3 lockfile.

If the lockfileVersion is explicitly set, we always use the version in
the options.

Also adds validation to the lockfileVersion config option, and cleans up
the logic in old/ancient lockfile inflation, so that we're not doing
that extra work if there just isn't a lockfile at all.

Finally, this also corrects a weird edge case where we were doing the
"inflate old lockfile" behavior when there is no lockfile present and we
load the initial idealTree state from the actualTree.  That would result
in adding integrity values for packages found present in the tree.
However, as we cannot be sure that `node_modules/foo` came from
`{registry}/foo`, this is an assertion we can't guarantee.  No one has
yet reported this obscure edge case as causing problems.