feature/featuretags: add build tag to remove captive portal detection

This doesn't yet fully pull it out into a feature/captiveportal package.
This is the usual first step, moving the code to its own files within
the same packages.

Updates #17254

Change-Id: Idfaec839debf7c96f51ca6520ce36ccf2f8eec92
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

cmd/tailscale/depaware.txt

@@ -103,7 +103,7 @@ tailscale.com/cmd/tailscale dependencies: (generated by github.com/tailscale/dep
         tailscale.com/envknob                                        from tailscale.com/client/local+
         tailscale.com/envknob/featureknob                            from tailscale.com/client/web
         tailscale.com/feature                                        from tailscale.com/tsweb+
-        tailscale.com/feature/buildfeatures                          from tailscale.com/cmd/tailscale/cli
+        tailscale.com/feature/buildfeatures                          from tailscale.com/cmd/tailscale/cli+
         tailscale.com/feature/capture/dissector                      from tailscale.com/cmd/tailscale/cli
         tailscale.com/feature/condregister/oauthkey                  from tailscale.com/cmd/tailscale/cli
         tailscale.com/feature/condregister/portmapper                from tailscale.com/cmd/tailscale/cli

cmd/tailscaled/deps_test.go

@@ -123,6 +123,19 @@ func TestOmitACME(t *testing.T) {
 	}.Check(t)
 }
 
+func TestOmitCaptivePortal(t *testing.T) {
+	deptest.DepChecker{
+		GOOS:   "linux",
+		GOARCH: "amd64",
+		Tags:   "ts_omit_captiveportal,ts_include_cli",
+		OnDep: func(dep string) {
+			if strings.Contains(dep, "captive") {
+				t.Errorf("unexpected dep with ts_omit_captiveportal: %q", dep)
+			}
+		},
+	}.Check(t)
+}
+
 func TestOmitOAuthKey(t *testing.T) {
 	deptest.DepChecker{
 		GOOS:   "linux",