RFR: 8321739: Source launcher fails with "Not a directory" error

[Christian Stein]

Please review this change to allow files sharing names with packages when using the Java source launcher.

The now used `!Files.exists()` returns a boolean value on Unix-based file systems instead of throwing an exception as `Files.notExists()` does when one or more elements of the tested path are files (and not directories), in addition of the the last element.

Tested: tier1

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Commit messages:
 - Source launcher fails with "Not a directory" error
 - Add failing test for JDK-8321739

Changes: https://git.openjdk.org/jdk/pull/17076/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=17076&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8321739
  Stats: 4 lines in 3 files changed: 1 ins; 0 del; 3 mod
  Patch: https://git.openjdk.org/jdk/pull/17076.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/17076/head:pull/17076

PR: https://git.openjdk.org/jdk/pull/17076