RFR: 8086: Some refactoring for jmc/core [v7]

[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 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 seven additional commits since the last revision:

 - Merge branch 'master' into coretestsrefac
 - Use maven.compiler.source for java version instead of hard coding
 - Don't touch checkformatting scripts in this change
 - Don't reorder core/pom.xml to ease reviewing
 - 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/0e54faed..b1954bcd

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jmc&pr=495&range=06
 - incr: https://webrevs.openjdk.org/?repo=jmc&pr=495&range=05-06

  Stats: 2909 lines in 48 files changed: 1389 ins; 1488 del; 32 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