Performance regression in bannedDependencies rule

Since adding maven-dependency-tree as a dependency, bannedDependencies has had a significant performance regression when it comes to building the dependency tree to check. In a multi-module project with more than one bannedDependencies rule defined, build times are significantly slower: one project using over 100 modules and 20 bannedDependencies rules takes 10 mins just to run maven validate, while using maven-enforcer-plugin 1.1.1 the build time is only 1 min.

The rule should be updated to cache the dependency tree for a project, and to reuse it across rules for the same project. To support parallel builds correctly, the cache will also have to support caching more than one dependency tree at a time.