RFR: 8261625: Add `Elements.isAutomaticModule(ModuleElement)` [v2]

[Joe Darcy]

> Addition of a small convenience method, with a default implementation, to Elements.
> 
> Most of the change is testing infrastructure. The negative cases, such as named modules, are tested in the style of other annotation processing tests for Elements utilities.
> 
> An automatic module comes from a jar file, so an API reading from a jar file is needed. To avoid piping in the awkward plumbing needed to do that directly in the annotation processing tests, I updated an existing automatic modules test which uses the tool box facility. My additions to the tool box might not be the done in the most idiomatic style for that API; happy to have guidance on how to proceed better there.
> 
> Assuming this general approach is agreed to, I'll update the imports on AutomaticModules.java to condense the text of the annotation processor source.

Joe Darcy 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 five additional commits since the last revision:

 - Respond to review feedback.
 - Merge branch 'master' into 8261625
 - Add test coverage.
 - Merge branch 'master' into 8261625
 - 8261625

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

Changes:
  - all: https://git.openjdk.java.net/jdk/pull/3382/files
  - new: https://git.openjdk.java.net/jdk/pull/3382/files/fdeadb26..5dab5c2f

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jdk&pr=3382&range=01
 - incr: https://webrevs.openjdk.java.net/?repo=jdk&pr=3382&range=00-01

  Stats: 2952 lines in 89 files changed: 2325 ins; 314 del; 313 mod
  Patch: https://git.openjdk.java.net/jdk/pull/3382.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/3382/head:pull/3382

PR: https://git.openjdk.java.net/jdk/pull/3382