Use case: an override module is designed to replace a built-in module
only when the module is required from an application, not from the
platform itself.

Now it is possible:

```lua
-- override/fio.lua
local loaders = require('internal.loaders')

if loaders.initializing then
    loaders.no_package_loaded.fio = true
    return loaders.builtin.fio
end

return {
    whoami = 'override.fio',
}
```

Follows up #7774

NO_DOC=Planned as an internal API. See the previous commit.
NO_CHANGELOG=see NO_DOC