RFR: 8271079: JavaFileObject#toUri and multi-release jars [v3]

[Christian Stein]

> Prior to this PR, `toUri()` of class `ZipPath` in module `jdk.zipfs` and class `PathFileObject` in module `jdk.compiler` were always composed by base path names. Even for versioned entries of a multi-release JAR file.
> 
> Now, a `URI` for an entry is composed of its real path names using an existing lookup function in the associated zip file system object.
> 
> This PR also removes a superseded work around for [JDK-8134451](https://bugs.openjdk.java.net/browse/JDK-8134451).
> 
> Fixes https://bugs.openjdk.java.net/browse/JDK-8271079

Christian Stein has updated the pull request incrementally with one additional commit since the last revision:

  Turn `getRealPath()` into a no-arg helper method

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

Changes:
  - all: https://git.openjdk.java.net/jdk/pull/6768/files
  - new: https://git.openjdk.java.net/jdk/pull/6768/files/e62721a5..12e106c6

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jdk&pr=6768&range=02
 - incr: https://webrevs.openjdk.java.net/?repo=jdk&pr=6768&range=01-02

  Stats: 5 lines in 1 file changed: 1 ins; 0 del; 4 mod
  Patch: https://git.openjdk.java.net/jdk/pull/6768.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/6768/head:pull/6768

PR: https://git.openjdk.java.net/jdk/pull/6768