Problem: `:checkhealth` fail to find the module when `&rtp` have nested
paths.
Solution: find in order all existed `&rtp/lua` path rather than `&rtp`
to ensure prefix exist before trim `&rtp`.
In this case one module can be searched out from two different
`&rtp/lua`, we use the first `&rtp/lua` contain the module (like how
require() works).