Add option to package list to access locally only and not check for updates.

[gitlost]

See https://github.com/wp-cli/package-command/issues/59#issuecomment-353336589

An option to not bother checking for updates to packages would be useful to avoid GITHUB_TOKEN and Composer auth issues like the above and is sometimes all one wants to know and would also be much much faster.

Might also consider combining with or having some local caching of update info.