A `module-suffixes` entry in a collection's "info.rkt" adds a
file suffix that is meant to be recognized globally (i.e., in
all collections) by all Racket tools.
The new fields are reported by `compiler/module-suffix` library, which
is (so far) used by `raco setup`.
Note that if package A includes files with a suffix that is registered
by package B, then A should have a dependency on B, but `raco setup`
cannot currently detect that such a dependency is needed. That
dependency is likely to happen, anyway, since package A is likely
using libraries form package B.
