RFR: 8338675: javac shouldn't silently change .jar files on the classpath [v6]

[David Beaumont]

> Modifying `JavacFileManager` to skip creating sibling output class files for source files found in JARs.
> 
> This should match older (JDK 8) behavior whereby the JAR was not writable, and results in any newly generated class files being written to the current working directory (the output of class files into current directory isn't good, but it should match the old behavior).

David Beaumont has updated the pull request incrementally with one additional commit since the last revision:

  Adding sanity check that the test directory is empty (since we clean it between tests).

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Changes:
  - all: https://git.openjdk.org/jdk/pull/23699/files
  - new: https://git.openjdk.org/jdk/pull/23699/files/2905c223..0a3613b6

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=23699&range=05
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=23699&range=04-05

  Stats: 12 lines in 1 file changed: 12 ins; 0 del; 0 mod
  Patch: https://git.openjdk.org/jdk/pull/23699.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/23699/head:pull/23699

PR: https://git.openjdk.org/jdk/pull/23699