RFR: 8217920: Lookup.defineClass injects a class that can access private members of any class in its own module [v3]

[Mandy Chung]

> Currently, a `Lookup` object with `PACKAGE` access can be used to inject a class in the runtime package of the Lookup's lookup class via `Lookup::defineClass`.   The classes that are injected have the same access as other members in the module and can access private members of all types in the module via reflection.
> 
> However, changing `Lookup.defineClass` to require full privilege access (`PRIVATE` + `MODULE`) is an incompatible change that would break existing frameworks which use `privateLookupIn` and `Lookup::defineClass` to inject auxiliary classes in a module.   A module authorizes the framework by opening a package for it to access and `Lookup::defineClass` was the supported replacement for `setAccessible` on `ClassLoader::defineClass` hack in JDK 9.    
> 
> This PR proposes to keep existing behavior and provide better documentation to help developers to beware of the permissions given out when opening a package to another module. A class injected in a module has the same privilege as other module members.

Mandy Chung has updated the pull request incrementally with one additional commit since the last revision:

  review feedback and add @apiNote

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Changes:
  - all: https://git.openjdk.org/jdk/pull/12236/files
  - new: https://git.openjdk.org/jdk/pull/12236/files/4d3e879c..487baca5

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=12236&range=02
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=12236&range=01-02

  Stats: 17 lines in 2 files changed: 11 ins; 0 del; 6 mod
  Patch: https://git.openjdk.org/jdk/pull/12236.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/12236/head:pull/12236

PR: https://git.openjdk.org/jdk/pull/12236