Rename "IMPORTED_CLOSEABLE" to "IMPORTED_PERMANENT" to clarify the meaning of the flag

Summary:
Ref T13591. This is an old flag with an old name, and there's an import bug because the outdated concept of "closable" is confusing two different behaviors.

This flag should mean only "is this commit reachable from a permanent ref?". Rename it to "IMPORTED_PERMANENT" to make that more clear.

Rename the "Unpublished" query to "Permanent" to make that more clear, as well.

Test Plan:

• Grepped for all affected symbols.
• Queried for all commmits, permament commits, and impermanent commits.
• Ran repository discovery.
• See also further changes in this change series for more extensive tests.

Maniphest Tasks: T13591

Differential Revision: https://secure.phabricator.com/D21514