RFR: 8218685: jlink --list-plugins needs to be readable and tidy [v5]

[Alan Bateman]

On Mon, 28 Sep 2020 23:05:37 GMT, Mandy Chung <mchung at openjdk.org> wrote:

>> Ian Graves has updated the pull request incrementally with one additional commit since the last revision:
>> 
>>   Additional usage narrowing
>
> Looks good to me.  It may be useful to have a regression test to verify the plugin order of the output if possible.

The new AbstractPlugin and each of the implementations extending it look good. The new usages messages look good too. A
few minor formatting nits in a few places but otherwise I think good to go.

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PR: https://git.openjdk.java.net/jdk/pull/305