RFR: 8335989: Implement Module Import Declarations (Second Preview) [v2]

[Jan Lahoda]

> This is a current patch for module imports declarations, second preview. At least the JEP number and preview revision will need to be updated in `jdk.internal.javac.PreviewFeature.Feature`, but otherwise I believe this is ready to receive feedback.
> 
> The main changes are:
> - `requires transitive java.base;` is permitted, under the preview flag. Both javac and the runtime module system are updated to accept this directive when preview is enabled.
> - the `java.se` module is using `requires transitive java.base;`, and is deemed to be participating in preview, so its classfile version is not tainted. Runtime is updated to access `requires transitive java.base;` in any `java.*`, considering all of them to be participating in preview. Can be tighten up to only `java.se` is desired.
> - the types imported through module imports can be shadowed using on-demand imports. So, for example, having:
> 
> import module java.base;
> import module java.desktop;
> ...
> List l;//ambigous
> 
> but:
> 
> import module java.base;
> import module java.desktop;
> import java.util.*;
> ...
> List l;//not ambigous, reference to java.util.List

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

  Reflecting review feedback.

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

Changes:
  - all: https://git.openjdk.org/jdk/pull/21431/files
  - new: https://git.openjdk.org/jdk/pull/21431/files/18554144..b7c871da

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=21431&range=01
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=21431&range=00-01

  Stats: 78 lines in 3 files changed: 37 ins; 7 del; 34 mod
  Patch: https://git.openjdk.org/jdk/pull/21431.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/21431/head:pull/21431

PR: https://git.openjdk.org/jdk/pull/21431