Moved API to internal directory

Author: steverao

instrumentation-api/src/test/java/io/opentelemetry/instrumentation/api/instrumenter/InstrumentationCustomizerTest.java renamed to instrumentation-api/src/test/java/io/opentelemetry/instrumentation/api/internal/InstrumentationCustomizerTest.java

@@ -3,7 +3,7 @@
  * SPDX-License-Identifier: Apache-2.0
  */
 
-package io.opentelemetry.instrumentation.api.instrumenter;
+package io.opentelemetry.instrumentation.api.internal;
 
 import static io.opentelemetry.sdk.testing.assertj.OpenTelemetryAssertions.assertThat;