Merge 'hs2py-constants' into 'hs2py'

Merge code of 'hs2py-constants' into 'hs2py', which requires adding
flag '--constants', and merge target 'src/hs2py-constants' into
'src/hs2py' in 'Makefile.am', also updating dependencies and
variables.

Signed-off-by: Jose A. Lopes <jabolopes@google.com>
Reviewed-by: Michele Tartara <mtartara@google.com>