Io: Provide directory existence with listing.

Sometimes, you need to tell the difference between an empty directory and
one that doesn't exist at all.  We can do this in a single call.

Core/FileSystems/MetaFileSystem.h

@@ -103,7 +103,7 @@ public:
 
 	std::string NormalizePrefix(std::string prefix) const;
 
-	std::vector<PSPFileInfo> GetDirListing(std::string path) override;
+	std::vector<PSPFileInfo> GetDirListing(const std::string &path, bool *exists = nullptr) override;
 	int      OpenFile(std::string filename, FileAccess access, const char *devicename = nullptr) override;
 	void     CloseFile(u32 handle) override;
 	size_t   ReadFile(u32 handle, u8 *pointer, s64 size) override;