[MPLUGIN-376] Drop legacy dependencies

[cstamas]

Plugin used a lot of legacy code, misused Mojo test case
and used old Junit 3.x TestCase, and missed some
dependencies (junit mostly).

Changes:

• drop use of Junit 3 TestCase, use Junit4 everywhere
• fix misuses of AbstractMojoTestCase when it is NOT mojo being tested
• add missing junit test dep where missing
• drop legacy p-c-d, sisu using exclusions/replacements
• align JavaMojoDescriptorExtractorTest.java for semantic difference: current Shim returns empty list instead of null when no components (for null safety). Assers updated but semantic is same: "there are no components discovered".

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Project cleaned up locally with help of apache/maven-parent#33
(locally modified parent and bashed POMs as long there were enforcer failures).

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This PR continues work done in #37 and will be merged AFTER IT.

[gnodet]

I wonder if we should switch to junit 5 rather than going to junit 4. It has been out for 4 years now. I've done such a migration a few times, so I can work on it when those PRs are merged.

[cstamas]

I am for it, but for now at least this PR aligns tests in this project, does not mix junit3 and junit4 anymore...