Improve structure and content of the examples Java sources

[pataluc]

Example source code for running tests located here: https://github.com/green-code-initiative/ecoCode-java/tree/main/src/test/files and copied (more or less) here: https://github.com/green-code-initiative/ecoCode-java-test-project/tree/main/src/main/java/fr/greencodeinitiative/java/checks could be vastly improved.

Problems:

• Files should probably be identical in both location, but are not (some have Licence in javadoc, some are under a java package, ...).
• Example source does not mentionned rule key, therefore it's often difficult to determine which serve to test which rule
• Example source often raise sonar issues that are not related to the one that is currently being tested: adds some noise to the tests

Suggestion:

• Find a way to store example source code once: by using git submodule maybe, or another mecanism.
• Correct use of the java packages, to tidy things up by adding a reference to the rule key: