Migrate all AOT tests to make use of `GeneratedClasses` rather than
directly generating Java files. This commit also refines and polishes
AOT APIs to being greater consistency.
Specifically:
- The `MethodGenerator` interface has been removed in favor of
working directly with `GeneratedMethods`.
- The visibility of several constructors and methods has been
reduced to package-private.
- The `using(...)` and `builder` methods have been removed in
favor of setting the `Consumer` callbacks directly as
constructor arguments.
- Variable names for builders are now named `type` or `method`
depending on what they're building.
Closes gh-28831
f2d31b7a20