Improve performance of transitive dependency checks

[harrisric]

Keep track of artifacts already validated during transitive dependency enforcement, so that we don't check the same ones multiple times.
In an example that I have we see the validation of ~19,000 artifacts where only ~1,300 are distinct.
This change keeps track of already checked artifacts and skips the validation.
I had tried to skip all the children of an already validated artifact but this did not achieve exactly the same result (some of the distinct artifacts no longer appeared in the validated set) and didn't actually seem to improve the performance much above the simple approach.
As Artifact (and specifically org.apache.maven.artifact.DefaultArtifact) doesn't include scope in its equals method I couldn't simply track a Set<Artifact> and created a class local to ArtifactMatcher for the purpose instead.

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@slawekjaranowski Please assign appropriate label to PR according to the type of change.