#24 Improve handling of conflicting modules

[Joachim Schrod]

Modules might be conflicting, the prototypical example
is word-order vs. letter-order. Where do we assert that
only one of these modules is loaded?
-- Not at all, remains the responsibility of the user.
-- At the texindy level, just for {word,letter}-order,
with special
option -l.
-- At the xindy level, hardwired for several modules.
-- At the module level, hardwired/hacked in the module
code.
-- At some kind of constraint specs to the xindy style
sheet
language to define module interfaces, a la DEFSYSTEM.