make `.nushell/` a proper module (!123)

this MR turns `./.nushell/` into a directory module by
- adding `mod.nu`
- exporting all the modules

all uses of `.nushell/` have been fixed to not mention `.nu` internal modules anymore.

>  **Note**  
> the `.nushell venv` module has been removed because, when the `$venv.VENV` activation script is not there, Nushell can't parse the whole `.nushell` module, which is very annoying to have to rely of the state of the external filesystem to be able to simply parse a module...