lua: refactor complete-filename plugin #518

Job	Run time
build (ubuntu-latest)	2m 37s
build (ubuntu-latest, --disable-curses)	2m 36s
build (ubuntu-latest, --disable-lua)	2m 18s
build (ubuntu-latest, --disable-tre)	2m 39s
build (ubuntu-latest, --disable-selinux)	2m 22s
build (ubuntu-latest, --disable-acl)	2m 30s
build (ubuntu-latest, --disable-help)	2m 35s
build (ubuntu-20.04)	2m 50s
build (ubuntu-20.04, --disable-curses)	2m 50s
build (ubuntu-20.04, --disable-lua)	2m 19s
build (ubuntu-20.04, --disable-tre)	2m 38s
build (ubuntu-20.04, --disable-selinux)	2m 20s
build (ubuntu-20.04, --disable-acl)	2m 22s
build (ubuntu-20.04, --disable-help)	2m 25s
	35m 21s

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