RFR: 8335989: JEP 494: Implement Module Import Declarations (Second Preview) [v6]

[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 with a new target base due to a merge or a rebase. The pull request now contains 20 commits:

 - Merge branch 'master' into JDK-8335989
 - Moving operators to the beginning of line, as suggested.
 - Updating PreviewFeature metadata
 - Cleanup.
 - Merge branch 'master' into JDK-8335989
 - Merge branch 'master' into JDK-8335989
 - Reflecting review feedback.
 - Cleanup.
 - Cleanup.
 - Fixing tests
 - ... and 10 more: https://git.openjdk.org/jdk/compare/a10b1ccd...21c835ca

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

Changes: https://git.openjdk.org/jdk/pull/21431/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=21431&range=05
  Stats: 762 lines in 28 files changed: 580 ins; 46 del; 136 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