Automake::Rule: expose suffix rules as a function, not a scalar

Automake::Rule: expose suffix rules as a function, not a scalar

This is just a preparatory refactoring in view of future patches.
No semantic change is intended.

* lib/Automake/Rule.pm ($suffix_rules): Turn from a package-level
variable to a lexical variable.
(suffix_rule): New function, expose the details of $suffix_rules
that are actually required by code outside thus modules --- and
only those details, no more.
(@EXPORT): Adjust.
* automake.in (derive_suffix): Likewise.

Signed-off-by: Stefano Lattarini <stefano.lattarini@gmail.com>