detectByResolvers: Don't mandate an actual file to be present
detectByResolvers: Don't mandate an actual file to be present

The main point of IOStream is to be able to use taglib with some content
that can't be fopen()'ed, for instance a network file, or a local file
on a system that doesn't support opening a file directly through fopen
& such.
This commit adds an overload for detectByResolvers that will stick to
using an IOStream all the way.

taglib/taglib,taglib/taglib,taglib/taglib