Found due to a LGTM warning about unused loop variable (related). As far
as I can tell the callback was sending fullname multiple times. KeyError
check added because I found NestedTest failed - mitogen.parent had
mitogen as one of it's related, and mitogen was not in the cache.
Refs #61
* Children should never generate a request for a module that has already
been sent, however there are a variety of edge cases where, e.g.
asynchronous calls are made into unloaded modules in a set of
children, causing those children to request modules (and deps) in a
different order, which might break deduplication. So add a warning to
catch when this happens, so we can figure out how to handle it.
Meanwhile it's only a warning since in the worst case, this just adds
needless latency.
* Don't bother treating sent packages separately, there doesn't seem to
be any need for this (after docs are updated to match how preloading
actually works now).
David Wilson