feat: Using dependency walking to verify JDK9 Plugin works

[Roiocam]

Motivation

solve #1040

Use dependency walk(inspired by sbt inspect) to detect JDK9 plugin works or not.

Benefits:

• No need to compile and package, which means no need unzip either.
• Fast detect(only few seconds)
• Could verify in one or all modules.

Usage

It is easier to use, with only one command.

sbt dependWalkerCheck

Verify It

You can verify this plugin works by using two ways:

• Rollback the change of Fix JDK9 plugin when using OSGi #1047, and then run command sbt jdk9Check
• enable this plugin on other jdk8-only module(i.e. actor), and then run command sbt actor/jdk9Check

[Roiocam]

@mdedetrich Do you have time to review this? It solves #1040 and is simpler.

[Roiocam]

Explain.

[Roiocam]

One more update, when i had apply this plugin check for all JDK9 modules, some of these complained CompileJdk9 wasn't in the tree.

It is a good signal because these modules do not have JDK9 classes to be packaged, or not even producing a jar.

[mdedetrich]

lgtm. My only complaint is that we now seem to have multiple JDK9 checks (@He-Pin also wrote one). While there technically isn't a problem with having multiple checks in this case it can be confusing for the readers of the code and I have a slight preference for this variant since its actually checking the root issue and importantly not checking for things that historically hasn't been issues

[He-Pin]

Should be fine, this is pre-checking and my PR is post-checking, we chatted on WeChat about how to check the jdk9 classes and come up with different solutions.

[He-Pin]

looks good to me, just naming thing.