Integrated: 8144891: ToolBox should use java.nio.file.Path internally, instead of java.io.File

[Christian Stein]

Please review this change set that gets rid of all direct `java.io.File` usages by introducing new constants in `ToolBox`, based on system property values.

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Commit messages:
 - Update test/langtools/tools/lib/toolbox/ToolBox.java
 - Update JavacTask.java
 - 8144891: ToolBox should use java.nio.file.Path internally, instead of java.io.File

Changes: https://git.openjdk.org/jdk/pull/14015/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=14015&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8144891
  Stats: 24 lines in 6 files changed: 5 ins; 8 del; 11 mod
  Patch: https://git.openjdk.org/jdk/pull/14015.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/14015/head:pull/14015

PR: https://git.openjdk.org/jdk/pull/14015