FindLua: Search for all library names in each path

Add the NAMES_PER_DIR option to our find_library call so that all names are considered in each path as we proceed through the search. This allows locally-built unversioned libraries to be found before versioned system libraries if they appear earlier in the set of paths to be searched.

Suggested on the mailing list:

• Sept 2018: FindLua.cmake module does not find a locally built lua library