make `.nushell/` a proper module

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...