Consider renaming mod to pkg

[kpcyrd]

mod seems slightly overloaded and/or counter-intuitive, since we claim to be a package manager we should consider changing the wording to package and pkg while keeping mod as an alias.

This is a somewhat invasive change, but something we should decide before further investing into the mod term.

[osintyes]

from the chocolatey windows is. :) @kpcyrd

[0x646e78]

I think this is decided @kpcyrd ? It's pkg on my install from master at least, but yes working is still around 'modules' and so would be good to be uniform in the wording.