Add 'offline' mode

[nachog00]

It might be worthwhile to add an offline mode. That is, to allow devs to shut off the fetcher logic completely.

If they are bringing in their own binaries, we should't be slowing them down with async fetching and potentially slow verifying of hashes on every build.

[ala-mode]

I agree. Verifying shouldn't take too long, but fetching can be resource intensive. Not sure about an exact design but maybe if (all?) binaries are specified, the user would get a message (maybe we should add a message for when we are fetching binaries too?) and bypass fetching completely. Also it makes sense to not fetch for any reason if the user passes an argument to accomplish this.

I think we would want a command line flag or similar. How can we hook our build script into such a technique?