RFR: 8198317: Enhance JavacTool.getTask for flexibility [v3]

[Jan Lahoda]

> This is a clone of https://github.com/openjdk/jdk/pull/1896, which got closed by Skara.
> 
> @lgxbslgx, please let me know if you would prefer to resubmit the PR yourself.

Jan Lahoda has updated the pull request incrementally with one additional commit since the last revision:

  Reflecting review comments.

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Changes:
  - all: https://git.openjdk.java.net/jdk/pull/2871/files
  - new: https://git.openjdk.java.net/jdk/pull/2871/files/ec3c8927..3ed19b95

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jdk&pr=2871&range=02
 - incr: https://webrevs.openjdk.java.net/?repo=jdk&pr=2871&range=01-02

  Stats: 3 lines in 2 files changed: 0 ins; 0 del; 3 mod
  Patch: https://git.openjdk.java.net/jdk/pull/2871.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/2871/head:pull/2871

PR: https://git.openjdk.java.net/jdk/pull/2871