refactor: move tunnel pkg down one level (#358)

* refactor: move tunnel pkg down one level

While there, reduce unnecessary dependency on external packages.

* file I forgot to commit

internal/tunnel/session_test.go

@@ -0,0 +1,17 @@
+package tunnel
+
+import "context"
+
+// MockableSession is a mockable session.
+type MockableSession struct {
+	// Result contains the bytes of the psiphon config.
+	Result []byte
+
+	// Err is the error, if any.
+	Err error
+}
+
+// FetchPsiphonConfig implements ExperimentSession.FetchPsiphonConfig
+func (sess *MockableSession) FetchPsiphonConfig(ctx context.Context) ([]byte, error) {
+	return sess.Result, sess.Err
+}