lint: Separate checkers by dependence on the internet.

lint: Separate checkers by dependence on the internet.

I think there are a couple of potential uses for this. It's somewhat a
separation in to what checkers are just checking the contents of the
repository (line length for example), and other checkers which are bringing in
external information which could change.

I'm thinking particularly, about treating network dependent checkers
differently when automatically running them, but this commit also adds a
--no-network flag to guix lint, which selects the checkers that don't access
the network, which could be useful if no network access is available.

* guix/lint.scm (%checkers): Rename to %all-checkers.
(%local-checkers, %network-dependent-checkers): New variables.
* guix/scripts/lint.scm (run-checkers): Make the checkers argument mandatory.
(list-checkers-and-exit): Handle the checkers as an argument.
(%options): Adjust for changes to %checkers, add a --no-network option, and
change how the --list-checkers option is handled.
(guix-lint): Adjust indentation, and update how the checkers are handled.