add a fetch command to propperly reenable preparation of networkless installs on remotes

[RonnyPfannschmidt]

prior to the changes wrt wheel caching and download cache management,
downloading exactly whats on pypi in order to later push it to a system with a different python version/abi was trivial and straightforward,

now the number of workarounds to have that "work" is skyrocketing

[rbtcollins]

This seems to be a dup of #2919

[RonnyPfannschmidt]

since --download is supposed to get deprecated, it might be sensible to go the way of providing a propper download command

[xavfernandez]

Looks like a dup of #2643, combined with #2965

[RonnyPfannschmidt]

yup, missed those