feature/condlite/expvar: add expvar stub package when metrics not needed

Saves ~53 KB from the min build.

Updates #12614

Change-Id: I73f9544a9feea06027c6ebdd222d712ada851299
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

cmd/tailscaled/deps_test.go

@@ -282,6 +282,7 @@ func TestMinTailscaledWithCLI(t *testing.T) {
 		},
 		BadDeps: map[string]string{
 			"golang.org/x/net/http2": "unexpected x/net/http2 dep; tailscale/tailscale#17305",
+			"expvar":                 "unexpected expvar dep",
 		},
 	}.Check(t)
 }