should be able to define reduce functions from backtick imports

[ccomparini]

I just had to write

@import 'json'.false_value
+false_value +{
    ...
}+

When I should have been able to write:

+``json``.false_value +{
    ...
}+`

The first gives error: Error src/optionizer/optionizer.fpl:40:2 near «``json``.false»: expected production name after '+'

json.false refers to a production just as well as a normal name would.