bug#25425: cannot express search path

[Ricardo Wurmus]

It seems that the “search-path-specification” cannot be used to include
placeholders such as “?” as required by LUA_PATH and LUA_CPATH.  The
search paths in Lua are not directories and they are not actual file
names.

I suggest adding an optional “placeholder” field to
“search-path-specification”, which would hold a string like “?/?.lua” in
this case.