RFR: 8325324: Implement Implicitly Declared Classes and Instance Main Methods (Third Preview) [v5]

[Jan Lahoda]

> This is an implementation of a change for JEP draft: Implicitly Declared Classes and Instance Main Methods (Third Preview)[1]. The primary change here is that implicitly declared classes have two new implicit imports:
> 
> import module java.base;
> import static java.io.IO.*;
> 
> 
> Both of these depend on other changes, the first one on https://github.com/openjdk/jdk/pull/18614, and the second one on https://bugs.openjdk.org/browse/JDK-8305457. This PR depends on 18614, and the patch here can should be able to work both in the presence and absence of `java.io.IO`, including testing. But, overall the patch should be fairly straightforward - the imports are injected at the same time the long-existing import for `java.lang` is added.
> 
> The JEP number if `jdk.internal.javac.PreviewFeature` will be adjusted once the JEP is moved to Candidate.
> 
> [1] https://openjdk.org/jeps/8323335

Jan Lahoda has updated the pull request incrementally with one additional commit since the last revision:

  Simplifying check for type existence, fixing bug with explicit imports.

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

Changes:
  - all: https://git.openjdk.org/jdk/pull/18633/files
  - new: https://git.openjdk.org/jdk/pull/18633/files/58017874..39561764

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=18633&range=04
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=18633&range=03-04

  Stats: 52 lines in 2 files changed: 42 ins; 2 del; 8 mod
  Patch: https://git.openjdk.org/jdk/pull/18633.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/18633/head:pull/18633

PR: https://git.openjdk.org/jdk/pull/18633