RFR: 8341184: Clean up the interaction between the launcher native code and the LauncherHelper [v3]

[Jaikiran Pai]

> Can I please get a review of this change, which simplifies the interaction between the `java` launcher's native code with the `sun.launcher.LauncherHelper`? 
> 
> As noted in https://bugs.openjdk.org/browse/JDK-8341184, this proposed change reduces the back and forth between the launcher's native code and the `LauncherHelper` class. This also removes the additional reflective lookups from the native code after the main class and main method have been determined by the `LauncherHelper`. 
> 
> Although this is a clean up of the code, the changes in the `LauncherHelper` to return a `MainEntry` have been done in a way to facilitate additional upcoming changes in this area, which propose to get rid of the JAR manifest parsing from the launcher's native code.
> 
> No new tests have been added. Existing tests in tier1, tier2 and tier3 continue to pass.

Jaikiran Pai has updated the pull request with a new target base due to a merge or a rebase. The incremental webrev excludes the unrelated changes brought in by the merge/rebase. The pull request contains three additional commits since the last revision:

 - merge latest from master branch
 - merge latest from master branch
 - 8341184: Clean up the interaction between the launcher native code and the LauncherHelper

-------------

Changes:
  - all: https://git.openjdk.org/jdk/pull/21256/files
  - new: https://git.openjdk.org/jdk/pull/21256/files/d451ea0b..3993eca5

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=21256&range=02
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=21256&range=01-02

  Stats: 109377 lines in 1998 files changed: 40932 ins; 62346 del; 6099 mod
  Patch: https://git.openjdk.org/jdk/pull/21256.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/21256/head:pull/21256

PR: https://git.openjdk.org/jdk/pull/21256