Simplify "builtin file" management and recover from races

Summary:
Fixes T11307. Fixes T8124. Currently, builtin files are tracked by using a special transform with an invalid source ID.

Just use a dedicated column instead. The transform thing is too clever/weird/hacky and exposes us to issues with the "file" and "transform" tables getting out of sync (possibly the issue in T11307?) and with race conditions.

Test Plan:

• Loaded profile "edit picture" page, saw builtins.
• Deleted all builtin files, put 3 second sleep in the storage engine write, loaded profile page in two windows.
  • Before patch: one of them failed with a race.
  • After patch: both of them loaded.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T8124, T11307

Differential Revision: https://secure.phabricator.com/D16271