Have just a single module that provides both the case sensitive and
insensitive operations. Turns out we hardly use the case insensitive
operations, and the places where we do are not performance sensitive
at all. So we use the PackageSet implementation which stores the
packages case sensitively and tack on the case insensitive operations
but with linear time implementations rather than log time. For the
merged module/type name use PackageIndex because that is what older
released versions exported, so less needless client breakage.