[Wilds] Remove include_path mangling and drop support for "externals/includes"
Ref T13098. This logic comes from D3243 and was theoretically used to let you install libphutil with Homebrew. Since libphutil no longer exists and the other use cases I can come up with are questionable/obsolete, remove it.
(As we move into T5055, I expect to provide better tools for bundling/managing external dependencies.)
Test Plan: Ran arc, same as the old arc.
Reviewed By: amckinley
Maniphest Tasks: T13098
Differential Revision: https://secure.phabricator.com/D19692