[GR-57939] Introduce Espresso external JVMCI and VMAccess.

[graalvmbot]

• 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.