[MDEP-590] Resolve classified POM artifacts from the reactor [4.0.x]

[wilx]

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Description

Backport of #12658 from master to maven-4.0.x.

ReactorReader currently returns the reactor project's build POM for every artifact whose extension is pom, without considering its classifier. Consequently, resolving an attached classified POM from the reactor returns pom.xml instead of the attached artifact.

This change limits the build-POM shortcut to unclassified POM requests. Classified POMs proceed through the existing artifact matching path, which selects the attached artifact by extension and classifier. Ordinary reactor POM and consumer-POM resolution remain unchanged.

The production change is patch-identical to #12658. The Core IT uses the File/String-based verifier API and suite-order registration required by the maven-4.0.x test harness.

Fixes apache/maven-dependency-plugin#1024

Tests

• Full root clean verify: all 38 modules passed with Maven 3.9.16 and Java 17.
• spotless:apply: no formatting changes.
• Focused MavenITmdep0590ClassifiedPomArtifactFromReactorTest against the freshly built Maven 4.0.0-SNAPSHOT distribution: 1 test, 0 failures, 0 errors, 0 skipped.