[Wilds] Remove libphutil

Summary:
Ref T13098. Historically, Phabricator was split into three parts:

• Phabricator, the server.
• Arcanist, the client.
• libphutil, libraries shared between the client and server.

One imagined use case for this was that libphutil might become a general-purpose library that other projects would use.

However, this didn't really happen, and it seems unlikely to at this point: Phabricator has become a relatively more sophisticated application platform; we didn't end up seeing or encouraging much custom development; what custom development there is basically embraces all of Phabricator since there are huge advantages to doing so; and a general "open source is awful" sort of factor here in the sense that open source users often don't have goals well aligned to our goals.

Turning "arc" into a client platform and building package management solidify us in this direction of being a standalone platform, not a standalone utility library.

Phabricator also depends on arcanist/. If it didn't, there would be a small advantage to saying "shared code + client for client, shared code + server for server", but there's no such distinction and it seems unlikely that one will ever exist. Even if it did, I think this has little value.

Nowadays, I think this separation has no advantages for us and one significant cost: it makes installing arcanist more difficult for end-users.

This will need some more finesssing (Phabricator will need some changes for compatibility, and a lot of stuff that still says "libphutil" or "phutil" may eventually want to say "arcanist"), and some stuff (like xhpast) is probably straight-up broken right now and needs some tweaking, but I don't anticipate any major issues here. There was never anything particularly magical about libphutil as a separate standalone library.

Test Plan: Ran arc, it gets about as far as it did before.

Reviewers: amckinley

Reviewed By: amckinley

Maniphest Tasks: T13098

Differential Revision: https://secure.phabricator.com/D19688