Render signature module tree separately from modules.
Render signature module tree separately from modules.

Signed-off-by: Edward Z. Yang <dbd597f5635f432486c5d365e9bb585b3eaa1853@cs.stanford.edu>

haskell/haddock,haskell/haddock,haskell/haddock,haskell/haddock,haskell/haddock