feature/buildfeatures: split const bools out of the featuretags package, add Has prefix

This renames the package+symbols in the earlier 17ffa80138 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>

feature/buildfeatures/buildfeatures.go

@@ -0,0 +1,10 @@
+// Copyright (c) Tailscale Inc & AUTHORS
+// SPDX-License-Identifier: BSD-3-Clause
+
+//go:generate go run gen.go
+
+// The buildfeatures package contains boolean constants indicating which
+// features were included in the binary (via build tags), for use in dead code
+// elimination when using separate build tag protected files is impractical
+// or undesirable.
+package buildfeatures