After this change, there is only 1 remaining dependency in Host to Target.
While this change might seem like kind of a heavy hammer to solving the dependency problem, it makes more sense even conceptually to be in Symbols. While some of the specific places to search for symbol files might change depending on the Host, this is not inherently true in the same way that "accessing the file system" or "starting threads" is fundamentally dependent on the Host.
PDBs, for example, recently became a reality on non-Windows platforms, and it's theoretically possible that DSYMs could become a thing on non MacOSX platforms (maybe in a remote debugging scenario). Other types of symbol files, such as DWO, DWP, etc have never been tied to any Host platform anyway.
So I think this change both makes sense logically, and also improves the layering.