The installed parsers are not lua modules, but they can be added to the package.cpath. Neovim searches for tree-sitter parsers in a parser directory on the runtimepath (:h rtp). Parsers installed with ...
You can follow this guide to install OpenResty on Ubuntu 20.04 if you have not yet installed it. If you do not have LuaRocks installed for OpenResty then install it ...