Hide NativeAPI service to apps launched by bootstraps

[alexarchambault]

This fixes errors such as this one, that can be seen when using jni-utils from an app launched by a coursier bootstrap:

java.util.ServiceConfigurationError: coursier.jniutils.NativeApi: coursier.bootstrap.launcher.jniutils.BootstrapNativeApi not a subtype
    java.base/java.util.ServiceLoader.fail(ServiceLoader.java:593)
    java.base/java.util.ServiceLoader$LazyClassPathLookupIterator.hasNextService(ServiceLoader.java:14)
    java.base/java.util.ServiceLoader$LazyClassPathLookupIterator.hasNext(ServiceLoader.java:13)
    java.base/java.util.ServiceLoader$2.hasNext(ServiceLoader.java:1309)
    java.base/java.util.ServiceLoader$3.hasNext(ServiceLoader.java:1393)
    coursier.jniutils.NativeApi.get(NativeApi.java:32)
    …

[alexarchambault]

Ideally, that would deserve a non-regression test… Maybe if / when I'll make it a bit more robust, by having this also hide the bootstrap own classes.

[alexarchambault]

That said, I'm not fond of hiding stuff with class loader hacks such as this… If ever users try to duplicate the class path by walking over the class loader hierarchy (and looking at each URLClassLoader say, then loading everything in a new distinct class loader hierarchy), they'll miss the loaders hiding stuff, and will get faulty duplicated class paths…