Make the --offline flag a little more offline

[f-f]

This PR stems from a note I left in here:

As an aside, we should patch memoisedGetPackageDependencies to respect the offline flag, since something that is not cached might hit the network. We can push the failures even lower in the stack, but I don't know off the top of my head if that would cause trouble (e.g. if we gate all the Spago.Registry functions with it). The Spago.Git module has it at least, we just haven't propagated in all the low-level places that hit the network since we usually handle it at high level.

..so this PR does just that. We add offline-ness checking a little deeper in the stack so that we don't accidentally hit the network.

[fsoikin]

Looks cool. I left some whenM/unlessM suggestions, but not really important, just reducing the boilerplace.

Also: what about testing this new behavior?

[f-f]

@fsoikin testing this behaviour would be cumbersome, as you would need to clear out the global spago cache, where the registry repos are stored. I don't think we do it anywhere in the test suite, and the location is OS-dependent, etc. I have attempted before and I didn't like it - I guess we could try again, but I also don't want to make it too hard to merge improvements just because we don't have the test infrastructure for it.