[MDEP-803] Preserve parent POMs when copying classified dependencies

[wilx]

With copy-dependencies, combining addParentPoms=true and classifier=sources attempts to resolve parent POMs as *-sources.pom. Those missing artifacts are silently omitted by default, or fail the build when failOnMissingClassifierArtifact=true.

Keep automatically added parent POMs separate from dependencies during classifier/type translation, while retaining inclusion/exclusion filters and overwrite handling. Explicit classified POM dependencies still work, including a POM that is also used as a parent. Clarify this interaction in the parameter documentation.

Fixes #1312 (MDEP-803).

Tests

The first commit adds an Invoker regression, verified failing on the missing project and dependency parent POMs before the second commit applies the fix. It covers parent ancestry, a parent also declared as a dependency, explicit classified POMs, a type override, exclusions, and repository layout.

Validated with Maven 3.9.16 and Java 21:

• mvn spotless:apply — clean.
• mvn -Prun-its clean install — 435 unit tests, no failures/errors, 1 existing skip; all 103 integration tests passed.
• Original standalone reproducer — both parent POMs copied in normal and strict modes; repeat execution with existing output also passed.

Checklist

• The pull request addresses one issue without unrelated changes.
• The description explains what changes, how, and why.
• Commits have meaningful subjects; the implementation commit includes the rationale.
• Add unit tests for the behavior change. Regression coverage is provided by a Maven Invoker integration test that fails without the fix.
• Run mvn verify — included in the successful clean install lifecycle.
• Run integration tests with -Prun-its.
• I hereby declare this contribution to be licenced under the Apache License Version 2.0, January 2004.
• In any other case, please file an Apache Individual Contributor License Agreement.