RFR: 8086: Some refactoring for jmc/core [v3]

[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

Christoph Langer has updated the pull request with a new target base due to a merge or a rebase. The incremental webrev excludes the unrelated changes brought in by the merge/rebase. The pull request contains three additional commits since the last revision:

 - Merge remote-tracking branch 'upstream/master' into coretestsrefac
 - Merge remote-tracking branch 'upstream/master' into coretestsrefac
 - JMC-8086

-------------

Changes:
  - all: https://git.openjdk.org/jmc/pull/495/files
  - new: https://git.openjdk.org/jmc/pull/495/files/8a95c491..4587e1bf

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jmc&pr=495&range=02
 - incr: https://webrevs.openjdk.org/?repo=jmc&pr=495&range=01-02

  Stats: 34 lines in 2 files changed: 34 ins; 0 del; 0 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