refactor: processing of input makefile rules

refactor: processing of input makefile rules

This is a pure refactoring, with no intended functional or semantic
changes.  It breaks up an overly-long function in three smaller
sub-functions.  This change will very especially useful for the work
on Automake-NG.

* lib/Automake/Rule.pm (define): Move quite a lot of code out, into ...
(_rule_defn_with_exeext_awareness, _maybe_warn_about_duplicated_target,
_conditionals_for_rule): ... these new subroutines.

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