RFR: 8368467: [lworld] Add new flag generation for jimage to support preview mode [v2]

[David Beaumont]

> Adds support for writing preview related flags into jimage files.
> 
> Preview mode is complex. It's not nearly as simple as "does something in /modules/xxx/... have an entry in /modules/xxx/META-INF/preview/...".
> 
> Specific issues include:
> 
> Supporting preview-only resources without forcing a double lookup on everything.
> Changing the set of entries in /packages/xxx directories to account for preview only packages in some modules.
> Minimising the work done during image reader initialization to only need to process the small number of preview resources (rather than scanning the whole file to look for them).
> The new flags added by this code address these issues, but calculating them correctly with only minor adjustments to the existing code was not feasible, it just became a lot larger and very complex.
> 
> To address this, a new type (ModuleReference) is introduced to track and then merge information about packages seen in each module. This allows a much simpler inner loop for processing resource paths when building the node tree, combined with a subsequent merging stage to produce the final package information for each module.
> 
> Not that since ModuleReference is needed during jimage reading, that class is already present in the previous PR on which this is based, but it starts to be used to calculate the module flags in this PR.
> 
> This PR can also adds the ImageReader unit tests for preview mode, which rely on being able to generate jimage files with preview mode flags in.
> 
> Compare and review this against https://github.com/openjdk/valhalla/pull/1619.

David Beaumont has updated the pull request incrementally with one additional commit since the last revision:

  Restoring lost changes and updating some comments.

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

Changes:
  - all: https://git.openjdk.org/valhalla/pull/1621/files
  - new: https://git.openjdk.org/valhalla/pull/1621/files/2d56562f..0c0ad6a3

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=valhalla&pr=1621&range=01
 - incr: https://webrevs.openjdk.org/?repo=valhalla&pr=1621&range=00-01

  Stats: 102 lines in 4 files changed: 9 ins; 48 del; 45 mod
  Patch: https://git.openjdk.org/valhalla/pull/1621.diff
  Fetch: git fetch https://git.openjdk.org/valhalla.git pull/1621/head:pull/1621

PR: https://git.openjdk.org/valhalla/pull/1621