Restore old expanded include path rules for workflows which fall through

Summary:
Ref T13490. Historically, "arc" was eager to load external libraries from many locations. I plan to make this significantly more structured/safer in the future (see T5055), but this rule got dropped as the "experimental" and "wilds" branches collapsed to "master".

Restore "next to arcanist" and "inside externals/includes" as valid search locations for classic workflows.

Test Plan: Ran "arc" commands in a working copy with a "load" library configured.

Maniphest Tasks: T13490

Differential Revision: https://secure.phabricator.com/D21004