feature/drive: start factoring out Taildrive, add ts_omit_drive build tag

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>

cmd/tailscaled/deps_test.go

@@ -61,3 +61,19 @@ func TestOmitReflectThings(t *testing.T) {
 		},
 	}.Check(t)
 }
+
+func TestOmitDrive(t *testing.T) {
+	deptest.DepChecker{
+		GOOS:   "linux",
+		GOARCH: "amd64",
+		Tags:   "ts_omit_drive,ts_include_cli",
+		OnDep: func(dep string) {
+			if strings.Contains(dep, "driveimpl") {
+				t.Errorf("unexpected dep with ts_omit_drive: %q", dep)
+			}
+			if strings.Contains(dep, "webdav") {
+				t.Errorf("unexpected dep with ts_omit_drive: %q", dep)
+			}
+		},
+	}.Check(t)
+}