RFR: 8224922: Access JavaFileObject from Element(s) [v13]

[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 incrementally with three additional commits since the last revision:

 - Merge pull request #5 from lahodaj/JDK-8224922-3
   
   Adding tests for unnamed module/package and automatic modules.
 - Using a specific class file as the source.
 - Adjusting to the specification requiring returning of the outtermost class' FileObject, adding tests for unnamed module/package and automatic modules.

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Changes:
  - all: https://git.openjdk.java.net/jdk/pull/5038/files
  - new: https://git.openjdk.java.net/jdk/pull/5038/files/1f5e548f..be167698

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jdk&pr=5038&range=12
 - incr: https://webrevs.openjdk.java.net/?repo=jdk&pr=5038&range=11-12

  Stats: 281 lines in 2 files changed: 277 ins; 0 del; 4 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