Espresso: Minor warning cleanups

Espresso: Move VM-side jvmci package to impl package
This avoids conflict with the guest JVMCI package

Native-Image: Simplify NativeImageClassLoader.initRemotePackageMap

Introduce VMAccess API
The VMAccess API can be used to retrieve JVMCI and Graal compiler providers that reflect the state of a JVM. VMAccess.Builder can be used to create such a JVM based on a class path, module path etc.

This commit adds 2 implementations of VMAccess.Builder that can be looked up via service loaders:

• A "host" VMAccess which reflects on the host JVM similar to how the native-image builder works today.
• A "espresso-context" VMAccess which creates and reflects on an espresso context.

The implementation of the espresso context vm access is partial and is done as a refactoring of Espresso's existing "internal" JVMCI implementation.

• On the JVMCI side, the main espresso JVMCI implementation classes are turned into abstract classes that are extended by 2 sublasses:
  one for internal JVMCI which works as before via substitutions of native methods;
  and one for the new external JVMCI which is implemented via polyglot interop.
• In espresso, some common code is moved from the existing substitution classes to some *Util classes. External JVMCI works over interop so some existing classes now implement interop and a number of new interop wrappers to util objects are introduced.