*nix systems can read any-case patch directories

Changes many patch_manager functions to use a case-less variant of
GetSubdirectory. Fixes patches not showing up on *nix systems when
patch directories are named with odd cases, i.e. `exeFS'.

src/core/file_sys/patch_manager.h

@@ -29,6 +29,11 @@ enum class TitleVersionFormat : u8 {
 std::string FormatTitleVersion(u32 version,
                                TitleVersionFormat format = TitleVersionFormat::ThreeElements);
 
+// Returns a directory with name matching name case-insensitive. Returns nullptr if directory
+// doesn't have a directory with name.
+std::shared_ptr<VfsDirectory> FindSubdirectoryCaseless(const std::shared_ptr<VfsDirectory> dir,
+                                                       const std::string& name);
+
 // A centralized class to manage patches to games.
 class PatchManager {
 public: