RFR: 8298601: Refactor archiving of java.lang.Module objects

[Ioi Lam]

This RFE is a prerequisite for [JDK-8296344: Remove dependency on G1 for writing the CDS archive heap](https://bugs.openjdk.org/browse/JDK-8296344). 

The main change of this RFE is in `HeapShared::archive_reachable_objects_from()`. We avoid the use of "archived objects" in the interface between CDS and the module system, and use integer "root indices" instead. (See note [1] in [JDK-8298600](https://bugs.openjdk.org/browse/JDK-8298600).)

Also, for better encapsulation, the module-specific code has been moved from `HeapShared::check_module_oop()` to the three new functions in modules.cpp.

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Besides the refactoring, this RFE also tightens up the code that deals with `java.lang.Module` oops and the corresponding C++ `ModuleEntry` objects.

As this is admittedly an obscured area of the CDS archive heap, I've added the following:

- Assertions to make sure that when an oop is archived, the corresponding C++ object is also archived, and vice-versa.
- More comments

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Commit messages:
 - 8298601: Refactor archiving of java.lang.Module objects

Changes: https://git.openjdk.org/jdk/pull/11715/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=11715&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8298601
  Stats: 208 lines in 8 files changed: 145 ins; 50 del; 13 mod
  Patch: https://git.openjdk.org/jdk/pull/11715.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/11715/head:pull/11715

PR: https://git.openjdk.org/jdk/pull/11715