RFR: 8086: Some refactoring for jmc/core

[Christoph Langer]

I worked on some improvements/refactoring for jmc/core. In detail:

Better handling of Spotless and Checkstyle:
    - Fix calling spotless/checkstyle directly in submodules by refering to a jmc.config.path
    - Integrate spotless into the regular build workflow

Cleanup of core/pom.xml:
    - Better order of elements
    - URL cleanups
    - Plugin version bumps
    - Use plugins section instead of pluginManagement because settings apply to the jmc project only and no other project would inherit

Lighter test projects:
    - They don't need to be p2 plugin projects since they only test basic functionality but not in the context of Eclipse/OSGI modules
    - Disable some maven plugin executions that are not needed there

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Commit messages:
 - JMC-8086

Changes: https://git.openjdk.org/jmc/pull/495/files
 Webrev: https://webrevs.openjdk.org/?repo=jmc&pr=495&range=00
  Issue: https://bugs.openjdk.org/browse/JMC-8086
  Stats: 971 lines in 52 files changed: 301 ins; 606 del; 64 mod
  Patch: https://git.openjdk.org/jmc/pull/495.diff
  Fetch: git fetch https://git.openjdk.org/jmc.git pull/495/head:pull/495

PR: https://git.openjdk.org/jmc/pull/495