do not rely on implicit conversion to `StringKey` and `StringKey::operator+` to concatenate string-like arguments

bug exposed by gaudi/Gaudi!1803 (merged), which limits the visibility of operator+ where one of the arguments is a StringKey -- the fact that that change causes Factory.cpp to no longer compile implies that the code was relying on an implicit conversion to StringKey. So change the code to no longer do that...

(note: this does not depend on gaudi/Gaudi!1803 (merged) -- on the contrary, it has to be merged prior to it)