Extension check checks for no parens not empty parens

[som-snytt]

The check for hidden extensions incorrectly tested for "empty params":

paramTps.isEmpty || memberIsImplicit && !methTp.hasImplicitParams || paramsCorrespond

but that intended to check for "no params" or "nilary" method. However, "nilary method" is a misnomer here, because the result type is "not even" a method type. (The nomenclature from Scala 2 is that a method with paramss that is Nil is "nilary", but List(Nil) is "nullary".)

The check is corrected and moved to the beginning of the method, now called memberHidesMethod. It is a rare early return.

Other calculated values are pushed into their use sites, in another mild refactor to aid reading and avoid calculating vals that won't be used.

If we had indentation syntax for &&, I would have avoided the early return, but I was not willing to add braces.

I removed the check file instead of updating line numbers because the information is repetitive. The interesting part is that it names the target type.

The test file omitted the case where the extension is nullary not nilary, so that is added.

Fixes #22821