Fixed problem with installAfter
Fixed problem with installAfter

The problem is that when an installAfter completely replaces a previous denotation, a symbol or NamedType
might still hang on to that denotation in the cache. We need to enforce that we switch to the new denotation.
This is done by setting the replaced denotation's validFor field to Nowhere.

reactormonk/dotty,VladimirNik/dotty,folone/dotty,vsalvis/dotty,spetz911/dotty,reactormonk/dotty,vsalvis/dotty,dotty-staging/dotty,AlexSikia/dotty,sjrd/dotty,densh/dotty,som-snytt/dotty,lampepfl/dotty,densh/dotty,dotty-staging/dotty,svalaskevicius/dotty,som-snytt/dotty,sjrd/dotty,lampepfl/dotty,dotty-staging/dotty,som-snytt/dotty,AlexSikia/dotty,dotty-staging/dotty,lampepfl/dotty,sjrd/dotty,lampepfl/dotty,auvik/dotty,angi2/dotty,yusuke2255/dotty,lampepfl/dotty,spetz911/dotty,sjrd/dotty,angi2/dotty,auvik/dotty,som-snytt/dotty,som-snytt/dotty,VladimirNik/dotty,svalaskevicius/dotty,folone/dotty,dotty-staging/dotty,sjrd/dotty,yusuke2255/dotty