Reuse getDependenciesToCheck results across rules

We have a build with a significant number of dependencies, and also a large number of <bannedDependencies/> rules. Together, they lead to a build where Enforcer takes tens of minutes.

Looking into bottlenecks, we found that AbstractBanDependencies#getDependenciesToCheck was taking a significant proportion of the build time (tens of percent). This method is called once per rule, so it's recalculating the dependencies for every rule across every execution.

Introducing a cache, to reuse the dependencies one they'd been calculated for a module, dropped the time spent in banned dependency enforcement by about 75%, which was reflected as a 60% improvement in the time of that build.

Our local implementation used the org.codehaus.plexus.context.Context to store the dependencies keyed by the concatenation of MavenProject#getId and searchTransitive; there may be a more appropriate scope to avoid edge cases with complex configurations.