This renames the package+symbols in the earlier 17ffa8013835d4 to be
in their own package ("buildfeatures") and start with the word "Has"
like "if buildfeatures.HasFoo {".
Updates #12614
Change-Id: I510e5f65993e5b76a0e163e3aa4543755213cbf6
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>