Optimize Exact Matcher Lookup for ModuleRulues

We observe a severe hot spot at ModuleRules.getRules() after 950 dependency
overrides are added into our project. This hot spot leads to 4x-5x slow down
at sbt update step. For details information, please refer to

http://stackoverflow.com/questions/22187264/dependencyoverrides-causes-sbt-update-times-to-go-up-by-4-5x