support for offline builds using system packages

[davide125]

Right now the sapling build downloads a number of packages from pypi and other places, which makes it unsuitable for packaging in most Linux distributions. Add support for disabling this logic and using system packages already installed on the host instead.

[pbar1]

@davide125 We're packaging this in NixOS with a similar concern expressed in this comment: https://github.com/NixOS/nixpkgs/pull/201798#discussion_r1028527680

[opticSquid]

Its more than a year this needs to be sorted out.

[muirdm]

We are planning to rework the Sapling open source build process. We will keep the offline build support in mind.

/cc @bigfootjon