Revert and change the apidoc and coverage dirs

Based on Michael's suggestion, this patch partially reverts my
changes. The new directories are:

- doc/api/py
- doc/api/hs
- doc/coverage/py
- doc/coverage/hs

Basically the Python-specific output moves one level down (into py/)
compared to the original location, and the Haskell stuff goes into
hs/.

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: Michael Hanselmann <hansmi@google.com>