Enable htools apidoc generation and unify dir names

Previously, Python api doc was under doc/api (which didn't match with
the target rule, apidoc). After this patch, we have the following:

- make py-apidoc generates Python api doc under doc/py-apidoc
- make hs-apidoc generates Haskell api doc under doc/hs-apidoc
- make apidoc does both (if hs-apidoc enabled at configure time)

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