Implement lazy loading for vim.submodule, this would be over-engineering
for inspect only, but we expect to use this solution also for more and
larger modules.
Instead of eager-loading during plugin/* sourcing, define runtime
modules such as `vim.inspect` as lazy builtins. Otherwise non-builtin
Lua modules such as `vim.inspect` would not be available during startup
(init.vim, `-c`, `--cmd`, …).
ref #6580
ref #8677
