RFR: 8306729: Add nominal descriptors of modules and packages to Constants API

[Adam Sotona]

Constants API already provides models for all loadable constants to help programs manipulating class files and modelling bytecode instructions. However no models of module and package constants are provided by Constants API. Every program manipulating class files must implement own models and validation of modules and packages constants.

This pul request adds `java.lang.constant.ModuleDesc` and `java.lang.constant.PackageDesc` to the Constants API. 

Classfile API will follow up and remove its internal implementations of `PackageDesc` and `ModuleDesc`. 

Please review this pull request and attached CSR.

Thank you,
Adam

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Commit messages:
 - 8306729: Add nominal descriptors of modules and packages to Constants API

Changes: https://git.openjdk.org/jdk/pull/13615/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=13615&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8306729
  Stats: 261 lines in 7 files changed: 252 ins; 2 del; 7 mod
  Patch: https://git.openjdk.org/jdk/pull/13615.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/13615/head:pull/13615

PR: https://git.openjdk.org/jdk/pull/13615