Summary: Ref T13490. This fixes one bug in D21025 (loading relative to "arcanist/" rather than the parent directory) and improves behavior when a library fails to load (we prompt the user to continue).
Test Plan:
- Ran `arc list` with bad library specifications, got a sensible error and an option to continue.
- Ran `arc list` with a library specification that existed next to "arcanist/", got a library load instead of an error.
Maniphest Tasks: T13490
Differential Revision: https://secure.phabricator.com/D21029
Summary: Ref T13395. Moves all remaining code in "libphutil/" into "arcanist/".
Test Plan: Ran various arc workflows, although this probably has some remaining rough edges.
Maniphest Tasks: T13395
Differential Revision: https://secure.phabricator.com/D20980
