add checkpoint_fullname(x::IndexEntry)

[oxinabox]

If we know a checkpoint's name expressed in the MODULE.SUBMODULE.NAME form
then right now you need to do
split(fullname)[1:end-1] == prefixes(x) && last(split(fullname)) == checkpoint_name(x)
to see if it matches some x from the index.

That is pretty gross.

We should add checkpoint_fullname(x::IndexEntry) = join(".", [prefixes(x); checkpoint_name(x)])
or something like that.
So that it can be checked easily.