Add pmd-compat6 module for maven-pmd-plugin

[adangel]

This makes it possible to use PMD7 with maven-pmd-plugin

Describe the PR

This is now a different approach, the idea is from @kohlschuetter (see #4609).

I could successfully run now maven-pmd-plugin 3.21.2 with PMD 7.0.0-rc4, by just adding this module as an additional dependency.

My plans for this module:

• Once this is merged, manually release just this module as version 7.0.0-rc4, so that it is available as net.sourceforge.pmd:pmd-compat6:7.0.0-rc4 in maven central for everybody.
• Then change the version of this module to 7.0.0-SNAPSHOT. It will be part of the main build.
• It will be released together with final 7.0.0 (or another rc, whatever comes first)
• It's only necessary, until maven-pmd-plugin is updated to use by default PMD 7. As changing defaults is considered a breaking change, the maven-pmd-plugin would also need a major version bump. In any case, with this module, we are decoupled now a bit more.
• We might already dump this module, after we released 7.0.0, not sure, how long we really want to maintain backwards compatibility. Maybe we should make this very clear.
• This module seems to be a good candidate to collect our @DeprecatedUntil700 annotated methods/classes.
• The module is intended to be used by maven-pmd-plugin users, but it might be useful in other contexts (although no guarantee is given, that it works)

Note: In no way, this module restores compatibility on the AST level - that means, that users have to migrate their rules/Xpaths in order to use PMD 7.

Some downsides of this approach:

• A couple of classes are duplicated now in pmd-compat6. If we change them in pmd-core, we should also change them now in pmd-compat6. pmd-compat6 contains a couple of integration tests to be sure, that maven can still execute PMD, so we should be able to catch the most obvious problems.

Advantages:

• We can update to 7.0.0-rc4 when running maven-pmd-plugin in our build. I tested it locally, and it worked.

Related issues

• Supersedes [core] Resurrect removed methods/classes for maven-pmd-plugin compat #4609
• Related to Does maven support PMD 7 (release candidates)? #4478
• Might be related to With the 7.0 RC Java API changes, how do I run against something other than a file? #4595
• Related to https://issues.apache.org/jira/browse/MPMD-379

Ready?

• Added unit tests for fixed bug/feature
• Passing all unit tests
• Complete build ./mvnw clean verify passes (checked automatically by github actions)
• Added (in-code) documentation (if needed)
• Release notes update
• Migration guide (this module is a shortcut and may allow to postpone the migration for a while)

[pmd-test]

	1 Message
📖	No regression tested rules have been changed.

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[adangel]

FYI - I'm going to merge this now and release net.sourceforge.pmd:pmd-compat6:7.0.0-rc4