I'm very slightly concerned that this may break silently when code is moved between libraries. Although this is rare, we've moved a decent amount of code from phabricator to libphutil in the past, and some between libphutil and arcanist, and a bit between arcanist and phabricator. Probably none of it actually called phutil_get_library_root(), but the scenario (where we move code that does) doesn't seem totally impossible to me.

If we're explicit with library names, the code will continue functioning correctly (or throw a very visible/obvious error, in the case of moving to a lower level library with the higher-level library not loaded). With implicit names, it may silently change the behavior of the code.

If this seems like a net positive even given that consideration, I'm fine with it. It seems pretty borderline to me (although I could also imagine library authors having a much easier time with this version of the function in the future, and facing nearly zero risk of cross-library code moves).