Use git-pkgs/pom for Maven effective-POM resolution

[andrew]

Replaces the local fetchPOM/mergePOMs parent-walk with github.com/git-pkgs/pom, which additionally interpolates ${...} properties, fills versions from <dependencyManagement>, and expands <scope>import</scope> BOMs. FetchDependencies now returns concrete versions (e.g. 2.17.2 instead of ${jackson.version}).

A clientFetcher adapter wraps core.Client so all resolver HTTP still goes through the existing rate limiting, retries and circuit breaker. The resolver and a CachingFetcher are held on Registry so repeated calls share parent/BOM fetches.

FetchPackage metadata (description, homepage, repository, licenses) now comes from pom.EffectivePOM, which does the same child-wins parent merge the old code did plus interpolation. FetchMaintainers keeps a minimal local <developers> parse since the pom library doesn't carry that element.

TestParentPOMResolution is extended to assert ${project.version} and a parent-defined ${lib.version} via dependencyManagement both resolve to concrete versions through FetchDependencies.

Net -39 lines.