[mpv] Implement robust sponsorblock solution

Used po5's simpler and more stable solution for getting the correct
configuration file directory to start the sponsorblock plugin on older
mpv installations.

mpv/.config/mpv/scripts/sponsorblock_legacy.lua

@@ -1,9 +1,3 @@
 if mp.get_script_directory == nil then
-    local scriptpath = debug.getinfo(1).short_src:match('.*/')
-    local originalpath = package.path
-    package.path = scriptpath .. "?.lua"
-
-    require 'sponsorblock/main'
-
-    package.path = originalpath
+  dofile(mp.find_config_file("scripts/sponsorblock/main.lua"))
 end