Call split-po-files on en-PT as well.

We were forgetting to do this!  The end result was that after
update-translations ran, we had a new en-PT.po file that was never
split into .rest.po and .datastore.po, so they were sticking around
with the old version.

Luckily we noticed because our build rule combined en-PT.po,
en-PT.datastore.po, and en-PT.rest.po into one big mega-file, and then
other parts of the system complained that msgid's were present twice
(in both en-PT.po and in one of the old split files).

Now I call split-po on en-PT as well.  To keep the data flow the same,
I separated out the split-po logic into a function, and just call that
function after downloading the en-PT file.

Auditors: james

Test plan:
Manually ran the new 'split_po en-PT' and saw it update
intl/translations/pofiles successfully, notably creating new
.datastore/rest files and deleting en-PT.po