The lua scripts hvextern.lua and hvextern-checkfile.lua of the hvextern package are in the .../source/hvextern/lualatex and .../source/hvextern/scripts directories and not where tex can find them, such as (I believe) in the .../tex/latex/hvextern or .../scripts/hvextern directories.
The lua scripts
hvextern.luaandhvextern-checkfile.luaof thehvexternpackage are in the.../source/hvextern/lualatexand.../source/hvextern/scriptsdirectories and not where tex can find them, such as (I believe) in the.../tex/latex/hvexternor.../scripts/hvexterndirectories.