RFR: 8328481: Implement Module Imports [v6]

[Jan Lahoda]

> This is an implementation of JEP JDK-8315129: Module Import Declarations (Preview). Please see the JEP for details:
> https://bugs.openjdk.org/browse/JDK-8315129
> 
> It is mostly straightforward - the module imports are parsed, and then expanded to import-on-demand in `TypeEnter`.
> There is a few notable aspects, however:
> - the AST node for import (`JCImport`) is holding the imported element as a field access, because so far, the imported element always had to have a '.' (even for import-on-demand). But for module imports, it is permissible to import from a module whose name does not have a dot (`import module m;`). The use of field access for ordinary import seems very useful, so I preferred to keep that, and created a new internal-only AST node for module imports. There is still only one public API AST node/interface, so this is purely an implementation choice.
> - JShell now supports module imports as well; and the default, implicit, script is changed to use it to import all of `java.base` if preview is enabled. It is expected that the default would be changed if/when the module imports feature is finalized.

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 module-imports
 - Including current module name as suggested.
 - Adding more tests for ambiguities.
 - Merge branch 'master' into module-imports
 - Merge branch 'master' into module-imports
 - Merge branch 'master' into module-imports
 - Fixing test.
 - Fixing disambiguation of module imports.
 - Fixing file name computation.
 - Merge branch 'master' into module-imports
 - ... and 10 more: https://git.openjdk.org/jdk/compare/03e84178...c75b8b7a

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Changes: https://git.openjdk.org/jdk/pull/18614/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=18614&range=05
  Stats: 1198 lines in 29 files changed: 1130 ins; 9 del; 59 mod
  Patch: https://git.openjdk.org/jdk/pull/18614.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/18614/head:pull/18614

PR: https://git.openjdk.org/jdk/pull/18614