common/fileutil: Convert namespace to Common::FS

Migrates a remaining common file over to the Common namespace, making it
consistent with the rest of common files.

This also allows for high-traffic FS related code to alias the
filesystem function namespace as

namespace FS = Common::FS;

for more concise typing.

src/common/logging/backend.h

@@ -94,7 +94,7 @@ public:
     void Write(const Entry& entry) override;
 
 private:
-    FileUtil::IOFile file;
+    Common::FS::IOFile file;
     std::size_t bytes_written;
 };