As of this commit (per the issue), the Taildrive code remains where it
was, but in new files that are protected by the new ts_omit_drive
build tag. Future commits will move it.
Updates #17058
Change-Id: Idf0a51db59e41ae8da6ea2b11d238aefc48b219e
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>