RFR: 8086: Some refactoring for jmc/core [v6]

[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 incrementally with one additional commit since the last revision:

  Use maven.compiler.source for java version instead of hard coding

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

Changes:
  - all: https://git.openjdk.org/jmc/pull/495/files
  - new: https://git.openjdk.org/jmc/pull/495/files/c4d896be..0e54faed

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jmc&pr=495&range=05
 - incr: https://webrevs.openjdk.org/?repo=jmc&pr=495&range=04-05

  Stats: 2 lines in 1 file changed: 0 ins; 0 del; 2 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