[lworld] RFR: 8368333: [lworld] Add preview mode to ImageReader and JRT file-system [v12]

[David Beaumont]

> Java changes for supporting preview mode when preview mode resources (with new location flags) are available.
> 
> At the moment, this code will operate on non-preview jimage files (1.0) and act as if no preview resources are available by virtue of the default value for missing attributes and package flags being zero (which matches jimage 1.0).
> 
> This should be reviewed on top of https://github.com/openjdk/valhalla/pull/1618

David Beaumont has updated the pull request with a new target base due to a merge or a rebase. The incremental webrev excludes the unrelated changes brought in by the merge/rebase. The pull request contains six additional commits since the last revision:

 - Merge branch 'lworld' into jdk_8368333_java/squashed
 - remove preview only entries when completing directories
 - remove ImageReaderFactory.java (again)
 - Reapplying feedback changes.
 - feedback changes
 - Resync after dependent PR submit.
   
   * Removing package root flag based on feedback.
   * Changing existing package flags during writing to match altered flag values.
   * Feedback changes, and fixing some comments.
   * Renaming slightly confusing "testEncoder" method.
   * Fixing unit tests to use new constructor.
   * Word smithing flags definitions.
   * Add workaround until new image writing code is in
   * Clarifying flag docs for /packages/xxx case
   * Java ImageReader changes for preview mode

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

Changes:
  - all: https://git.openjdk.org/valhalla/pull/1619/files
  - new: https://git.openjdk.org/valhalla/pull/1619/files/8de11a9e..78f78354

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=valhalla&pr=1619&range=11
 - incr: https://webrevs.openjdk.org/?repo=valhalla&pr=1619&range=10-11

  Stats: 32218 lines in 890 files changed: 20120 ins; 8765 del; 3333 mod
  Patch: https://git.openjdk.org/valhalla/pull/1619.diff
  Fetch: git fetch https://git.openjdk.org/valhalla.git pull/1619/head:pull/1619

PR: https://git.openjdk.org/valhalla/pull/1619