RFR: 8224922: Access JavaFileObject from Element(s) [v9]

[Joe Darcy]

> Initial review to get some comments on the shape of the API. Tests are needed of course, as well as some tuning of the spec to better describe differences in behavior when class files rather than source files are the backing file type.

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 26 additional commits since the last revision:

 - Update spec for unnamed packages and modules.
 - Merge branch 'master' into JDK-8224922
 - Merge branch 'master' into JDK-8224922
 - Merge branch 'master' into JDK-8224922
 - Merge branch 'master' into JDK-8224922
 - Merge branch 'master' into JDK-8224922
 - Ancillary improvements to the spec.
 - Merge branch 'master' into JDK-8224922
 - Merge branch 'master' into JDK-8224922
 - Appease jcheck.
 - ... and 16 more: https://git.openjdk.java.net/jdk/compare/74fcc8f7...1d2492d7

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

Changes:
  - all: https://git.openjdk.java.net/jdk/pull/5038/files
  - new: https://git.openjdk.java.net/jdk/pull/5038/files/1599509b..1d2492d7

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jdk&pr=5038&range=08
 - incr: https://webrevs.openjdk.java.net/?repo=jdk&pr=5038&range=07-08

  Stats: 433903 lines in 959 files changed: 221227 ins; 199726 del; 12950 mod
  Patch: https://git.openjdk.java.net/jdk/pull/5038.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/5038/head:pull/5038

PR: https://git.openjdk.java.net/jdk/pull/5038