modules/http: fix finding the static files

I also verified there's no other usage of the `moduledir` symbol from
lua.  Bug introduced in 2f81b111 (within !298).
5 jobs for <span class="ref-name">http-moduledir</span> in 13 minutes and 28 seconds (queued for 3 seconds)
Status Job ID Name Coverage
  Build
canceled #73003
docker linux amd64
build:linux:amd64

passed #15974
docker linux amd64
build:linux:amd64

00:00:34

 
  Test
passed #15976
docker linux amd64
deckard:linux:amd64

00:00:32

canceled #15977
docker linux amd64 allowed to fail
respdiff:linux:amd64

00:13:27

passed #15975
docker linux amd64
test:linux:amd64

00:00:22