refactor: move Acceleo templates for Java to java subpackage

[petrhejcl]

Description

Move all existing Acceleo templates for generating Java code from org.eclipse.lyo.oslc4j.codegenerator to org.eclipse.lyo.oslc4j.codegenerator.java subpackage as a preparation for adding templates for other generating code for different languages.

Checklist

• This PR adds an entry to the CHANGELOG. _See https://keepachangelog.com/en/1.0.0/ for instructions. Minor edits are exempt.
• This PR expects some manual changes to the generated code. The needed changes are detailed in the CHANGELOG entry.
• This PR was tested with at least one code generation, resulting in a working OSLC server.
• This PR does NOT break the user block code

Issues

Closes #0; Closes #00

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