RFR: 8086: Some refactoring for jmc/core [v8]

[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` variable
>     - Integrate spotless into the regular build workflow
> 
> Cleanup of core/pom.xml:
>     - URL cleanups
>     - Plugin version bumps
> 
> 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 refreshed the contents of this pull request, and previous commits have been removed. The incremental views will show differences compared to the previous content of the PR. The pull request contains one new commit since the last revision:

  JMC-8086

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Changes:
  - all: https://git.openjdk.org/jmc/pull/495/files
  - new: https://git.openjdk.org/jmc/pull/495/files/b1954bcd..3f197864

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jmc&pr=495&range=07
 - incr: https://webrevs.openjdk.org/?repo=jmc&pr=495&range=06-07

  Stats: 0 lines in 0 files changed: 0 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