This fixes a startup crash in helpers.maybe_require() on mpv builds that use a Lua 5.1-compatible runtime.

maybe_require() currently calls package.searchpath(), but that API is not available on Lua 5.1/LuaJIT. On those builds, mpvacious crashes during startup when loading the optional custom_subtitle_filter module.

Reproduction

• macOS
• Homebrew mpv
• mpv links against a Lua 5.1-compatible runtime
• startup fails with:

attempt to call field 'searchpath' (a nil value)

Reference:

• Homebrew mpv formula: depends_on "luajit" and builds with -Dlua=luajit
  https://github.com/Homebrew/homebrew-core/blob/master/Formula/m/mpv.rb
• Homebrew luajit formula:
  https://github.com/Homebrew/homebrew-core/blob/master/Formula/l/luajit.rb

Fix

• keep using package.searchpath() when available
• add a small fallback for Lua 5.1-compatible runtimes that resolves path templates by replacing ? and checking file existence