Move `PackageTemplate` merging decisions entirely into `package.rs`

Move `PackageTemplate` merging decisions entirely into `package.rs`

Currently our package merging logic is strewn about between
`package.rs` (which decides various flags based on the package
type) and `onchaintx.rs` (which does the actual merging based on
the derived flags as well as its own logic), making the logic hard
to follow.

Instead, here we consolidate the package merging logic entirely
into `package.rs` with a new `PackageTemplate::can_merge_with`
method that decides if merging can happen. We also simplify the
merge pass in `update_claims_view_from_requests` to try to
maximally merge by testing each pair of `PackateTemplate`s we're
given to see if they can be merged.

This is overly complicated (and inefficient) for today's merge
logic, but over the coming commits we'll expand when we can merge
and not having to think about the merge pass' behavior makes that
much simpler (and O(N^2) for <1000 elements done only once when a
commitment transaction confirms is fine).